BEING A KID in Washington has its advantages: a wealth of free museums, acres of parks, stages full of kid- oriented entertainment and an assortment of trolleys, paddleboats and carousels to ride on. And most of it's free or at least affordable. Herewith, a sampling of Weekend's Best for Kids:

GETTING A GRIP ON ANIMALS LARGE. . . -- See the lions, the tigers, the pandas (aim for 11 and 3 when they're fed and sure to be awake) and the elephants (demonstrations are at 11:30 most days). But don't forget Herplab, open noon to 3 Wednesday through Sunday, where you can check out a box containing a live snake or frog of the day and examine it up close, or put together a tortoise-shell puzzle, or watch a movie starring the zoo's own lizards. There's also a Birdlab open noon to 3 Friday through Sunday and, brand-new, the Invertebrate Exhibit featuring up-close looks at spineless creatures, Thursday through Sunday. It's all free, but parking's $3. 3001 Connecticut Ave. NW, near the Woodley Park-Zoo station on Metro's Red line. 673-4800.

. . . AND SMALL -- At the Insect Zoo at the National Museum of Natural History, you can pet a live Madagascar hissing cockroach or stroke a fat green tobacco hornworm. But the stars of the Insect Zoo are the scary scorpions and the hairy tarantulas (fed on weekends at 11:30. 12:30 and 1:30). It's open 10 to 5:30 daily except Christmas. 357-2700.

A WHOLE MUSEUM JUST FOR KIDS -- There's always something new at the Capital Children's Museum. Along with the ever-popular Mexican village where you can make your own tortillas, drink hot chocolate and make paper flowers and necklaces, there are computers to work on, mock sewers to crawl through, telephones to dial, a grocery store to play in and special exhibitions and puppet shows, storytelling and other weekend performances. And there's a fantasy playground outside and, at the entrance, an even more fantastical indoor village peopled with figures made of cloth, broken glass, beads and bottle tops by Nek Chan, an artist from India. It's open 10 to 5 daily, 800 Third St NE. Museum only is $4, museum and a show is $6. 543-8600.

JUST FOR KIDS -- The Discovery Room at the National Museum of Natural History is just about the best hands-on museum experience in town. Check out a drawer full of shells or bones or insects, or peer into jars of preserved animal specimens. It's free but popular, so you have to pick up a pass at the Discovery Room door for your 30-minute time allotment. It's open noon to 2:30 Monday to Thursday, 10:30 to 3:30 Friday through Sunday. 357-2700.

BACK (IN TIME) ON THE FARM -- City kids don't get a chance to milk a cow very often, but at Oxon Hill Farm there are cows to milk, pigs to oink at and other animals to stare at over a fence. The farm's run by the National Park Service to show what rural life was like at the turn of the century. It's low-key but always fun with simple pleasures like rolling in the hay or going on a hayride. Year round, the farm holds festivals and demonstrations (the biggest is the Harvest Festival). It's free and open daily 8:30 to 5, on Oxon Hill Road (Beltway Exit 3A, right on Oxon Hill Road). 839-1177.

NATURE FOR EAGER BEAVERS -- Almost any Nature Center in the Washington area is a good bet for a family outing. But at Virginia's Huntley Meadows Nature Center, you can stroll along a mile-long boardwalk through a marshland filled with beaver and muskrat lodges, bullfrogs and other watery wildlife. There's a wooden tower visitors can climb to get a good look at beavers building a dam. It's a bird-watcher's paradise, too -- geese, ducks, even an ibis or two. It's free; the park is open sunrise to sunset and the nature center 9 to 5 weekdays, noon to 5 weekends, at 3701 Lockheed Blvd. in Hybla Valley. 768-2525.

A TAME WALK ON THE WILD SIDE -- Some of the best family hikes around are at Riverbend Park in Great Falls, Virginia. For pre-schoolers, there's the Duff 'n' Stuff trail, a quarter-mile stroller-friendly paved route with eight stops to look for natural treasures. You can call ahead and reserve a Discovery Bag for pre-schoolers that includes a squirrel puppet, sniff boxes and pictures keyed to stations along the trail.

For longer legs (kindergarten and up), there's the 1 1/4-mile Paw Paw Passage, my family's favorite. Winding through leafy sun-speckled woods, the rarely crowded trail takes you over bridges, hills and dales, with lots of discoveries along the way: tree fungus, mushrooms, even a deer's jawbone on one walk. The trail comes out on the Potomac shore and you can either loop back to Riverbend or continue south along the river to watch the kayakers shooting the rapids at Great Falls. Riverbend Nature Center is open 9 to 5 weekdays, and Saturdays and Sundays noon to 5, closed Tuesdays. The park is open dawn to dusk and there are usually trail guides in the information board outside the center. It's at 8814 Jeffery Rd., Great Falls, Virginia. 759-3211. No charge at the Nature Center but there is a park charge for non-county residents at the Visitor Center.

STARRY, STARRY SKIES -- The mysteries of the heavens are revealed every weekend at the Rock Creek Planetarium. And you don't even have to stay up past your bedtime. Two free shows start promptly at 1 and 4 (no latecomers admitted). The first show, "An Introduction to the Night Sky," is for ages 4 and up and the 4 o'clock show, for ages 7 and up, is all about Neptune. Free tickets are handed out a half hour before the show, at 5200 Glover Road NW. 426-6829. (The Planetarium has been closed for renovations but should re-open up the last week of October.)

ONCE UPON A TIME -- There's hardly an hour goes by that there isn't a story being told to kids somewhere in the Washington area, usually at the local library. And of course, there's always Dial-a-Story, the 24-hour story hotline operated by D.C.'s Martin Luther King Library (call 638-5717). But two of the best storytelling venues are down on the Mall. At the new National Museum of African Art, the tradition of Saturday afternoon storytelling sessions has started up again (call 357-4860 for schedule). And at the Arthur M. Sackler Gallery, kids can hear near- and far-east tales right in the galleries, every second Saturday, at 9:30 and 5:30 (call 357-4886 to make reservations). The stories are free and both museums are on the Mall, behind the Smithsonian Castle.

ESCAPING SATURDAY MORNING CARTOONS -- Say goodbye to "Fraggle Rock" and hello to live music, dance, magic, vaudeville, puppet shows and plays, just a few of the offerings of Saturday Morning at the National. It's a series of theatrical events for children 4 and up, presented every Saturday morning throughout the school year. Shows are at 9:30 and 11; they're free but you should reserve one week in advance by calling 783-3372. The National Theater is at 1321 Pennsylvania Ave. NW.

KIDDIE-CLASS AIR FARE -- You don't even have to leave home Saturday mornings to find an alternative to TV cartoons. Just turn off the boob and tune the radio to Children's Radio Theater at 8 a.m. on WPFW-FM (89.3) for an exciting radio play that will exercise kids' imagination muscles. Kids can also enter the year-end annual Henny Penny Playwriting Contest and get the chance to have their play produced on the show. Tune in for details in late November.

TOP OF THE PLAYGROUNDS -- Whether you live in Washington, Maryland or Virginia, playgrounds abound. Here are our three favorites: In Virginia, Abingdon Elementary School, where the play equipment is designed in the shape of the United States (the high climbers are the Rockies, the alligator is Florida). It's behind Abingdon School, 3035 South Abingdon St. in Arlington. 845-7664. In Maryland: Cabin John Regional Park, with five acres of equipment including a fort, a castle, tepees and Noah's Ark, a small domestic animal zoo. The park is on Tuckerman Lane in Bethesda and it's open 10 a.m. to sunset. In Washington: Kenilworth Parkside Recreation Center in Northeast has ripple slides, merry-go-rounds, a circular slide, swings for tots and older kids, all within sight of the Anacostia River. It's at 4300 Anacostia Dr. NE. 767-7345.

TURNING AND TURNING -- Feel like making a grab for the brass ring? Kids can mount one of 60 trusty steeds on the carousel in the shadow of the Smithsonian Castle on the Mall. It's open 10:30 to 5 on weekends, 10:30 to 4:30 weekdays, May to September, depending on weather. A three-minute ride costs 75 cents. The other big carousel in town is the 66-year-old Dentzel carousel in Glen Echo Park. It's open 10 to 2 Wednesdays and noon to 6 weekends, early May until the end of September. Rides are still 25 cents.

ZING THEIR HEARTSTRINGS -- Not so very long ago, Washington had one heck of a streetcar system. Offering a little taste of what we've been missing ever since the trolley to Glen Echo was torn up is the National Capital Trolley Museum. There are 15 vintage trolleys owned and operated by the museum, a labor of love by volunteers whose reward consists mainly of sharing the delight of young visitors. The museum is free, trolley rides are $1 adult, 75 cents for kids 3 to 18, 2 and under free. It's open weekends noon to 5 (last trolley leaves at 4:30) year round and noon to 4 Wednesdays during July and August. It's on Bonifant Road off New Hampshire Avenue extended in Wheaton. 384-9797.

BATTLE STATIONS -- Museum, adventure and playground all in one, the Barry (DD 933), a decommisioned Navy destroyer now on permanent duty at the Washington Navy Yard, is a would-be sailor's delight. Kids can poke their noses into just about every corner of the ship -- mess, galleys, officers quarters, engine rooms, guns, even sit in the captain's chair. It's open for self-guided tours 10 to 5 daily, at the Washington Navy Yard, Ninth and M SE. While you're there, don't miss the Navy Memorial Museum nearby where kids can climb on anti-aircraft guns and peer through periscopes. It's open 9 to 5 weekdays, 10 to 5 weekends and holidays. 433-2651.

KEEPING AFLOAT -- There are lots of ways to weigh anchor onto the waters in and around Washington, but the perennial favorite of kids is pedaling a paddleboat on the Tidal Basin. Rent them for $5.15 per hour at the Tidal Basin Boat House, 15th Street and Maine Avenue SW. The season runs end of March through the end of October, Wednesday through Sunday 10 to 6. 889-3800.

NOT ONLY ON SUNDAYS -- Take the kids to church on Saturday for a change, to the National Cathedral, for instance, to see gothic gargoyles and a moon rock embedded in the stained-glass window dedicated to space travel. Tours of the cathedral, at Massachusetts and Wisconsin NW, are 10 to noon and 1 to 3:15 Monday through Saturday, and Sunday afternoons at 1 and 2. 537-6200. Or head Northeast to the Franciscan Monastery and Gardens, 14th and Quincy streets NE. It's a 40-acre refuge including a Byzantine church modeled after St. Sophia Church in Istanbul, and a copy of the catacombs in Rome, plus gardens and grottos. Free tours are hourly from 9 to 4 except at noon, Monday through Saturday and 1 to 4 Sunday. Come Christmas, take the kids to the Mormon Temple. Although nonMormons are not allowed inside, the Christmas lights on the gleaming white towers and all around the grounds are spectacular. It's at 9900 Stoney Brook Dr. in Kensington, just off the Beltway.