Fun can't get much cheaper this Saturday than at the Greater Laurel Nursing Home's "Second Annual Carnival." For just 20 cents kids 12 and under can either buy cotton candy, throw darts at a balloon, pick a lollipop from a tree or a duck from a pond, toss a penny in a floating plate or play other games galore for prizes.

Speaking of games, a hundred years ago kids couldn't just bop down to the neighborhood video arcade and veg out on Pac Man for the day -- they had to create their own fun. On Sunday at Glen Echo's Clara Barton Historical Site, kids ages eight to 12 can learn how children of the late-1800s used household items and a bit of imagination to entertain themselves. While you're there, take a free tour of the original home of the Red Cross and learn what a kid's life was like a century ago.

All events are free unless otherwise noted.

NATIONAL ZOO -- Open daily; gates open from 8 to 8; animal buildings, 9 to 6; panda feedings, 11 and 3; elephant training demonstrations, 2; Sea lion training demonstrations, 2, 3001 Connecticut Ave. NW (Rock Creek Park), on the Metro Red Line. Family learning centers Zoolab and Birdlab open Friday through Sunday, noon to 3; Herplab open Wednesday through Sunday, noon to 3. 673-4717.

NATIONAL AQUARIUM -- Open daily, 9 to 5, Department of Commerce, 14th and Constitution Avenue NW. The sharks are fed Monday, Wednesday and Saturday at 2; weekday features are a touch-tank, mini- theater and a thousand types of marine life. School groups should schedule in advance. Adults $1; children 50 cents. 377-2825.

CEDAR HILL -- Open daily, 9 to 4, 1411 W St. SE, Anacostia. Cedar Hill, Frederick Douglass' home, recalls his work with films, interpretive exhibits and a house tour. 426- 5961 or 426-5962 or 426-5963.

USS BARRY -- Open daily, 10 to 5, Washington Navy Yard, Ninth and M streets SE. The Navy destroyer's weather deck is open for public viewing and self-guided tours. 433-3377.

CHILDREN'S MUSEUM -- Monday through Saturday, 10 to 4, Sunday 1 to 4, Rose Hill Children's Museum north of Frederick, Maryland. Take U.S. 15 north to Exit 8 to Motter Avenue; follow Motter to left onto 14th Street and left onto North Market Street, follow signs to Rose Hill. Costumed tour guides conduct tours of the 1790 mansion. Hands-on exhibits include a loom, antique toys and spinning wheel. Adults $1; under 17 free. 301/694-1646.

CAPITAL CHILDREN'S MUSEUM -- Daily, 10 to 5, 800 Third St. NE. Friday, 10:30; Saturday, 11, "Puppets on Parade." A variety show of colorful hand, rod and mouth puppets that perform musical numbers. Saturday, 2; Sunday, 3, "Spirit of Magic." Nestor the Magician saws people in half and makes animals disappear into thin air. Saturday and Sunday, museum hours, "Make Smoke Paper." Learn how to use paint, water and a special drying process to make smoky-looking paper art. Museum admission, $2; special performances an additional $2. 543-8600.

DOLL'S HOUSE AND TOY MUSEUM -- Tuesday through Saturday, 10 to 5; Sunday, noon to 5, 5236 44th St. NW. A collection of antique dollhouses, dolls, toys, games and an exhibit of antique cat games and toys. Adults $2; under 14, $1; plus group rates. 244-0024.

NATIONAL COLONIAL FARM -- Tuesday through Sunday, 10 to 5, 3400 Bryan Point Rd., Accokeek. Sunday, 1 to 4, "Colonial Needlework." Staff members demonstrate various forms of needlework, including quilting, smocking and embroidery. Adults $1; children under 12, free. 301/283-2113.

CLOISTERS CHILDREN'S MUSEUM -- Wednesday through Friday, 10 to 4:30; Saturday, 12 to 4, 10440 Falls Rd., Brooklandville, Maryland, "Indian Summer Celebration." Hunt bear and porcupine hidden along a nature trail, dance around a totem pole and see an extensive Kachina doll exhibit. Saturday, museum hours, "Indian Crafts": make tepees, sandpaintings and decorate Indian headbands. $2 donation requested from adults; group tours available. 301/823-2551.

RAILROAD STATION MUSEUM -- Wednesday through Saturday, 11 to 4; Sunday, noon to 5, Main Street and Maryland Avenue, Ellicott City. The renovated station has a diorama of the trip of Tom Thumb (an early 19th-century train) from Baltimore to Ellicott City. There are also sight and sound shows, model train exhibits and tours of the museum. Adults $1.25; under 13, 75 cents. 301/461-1944.

COLONIAL FARM -- Wednesday through Sunday, 10 to 4:30, Claude Moore Colonial Farm at Turkey Run, 6310 Old Georgetown Pike, McLean. Take a self-guided tour of farm life for a low-income family in Colonial Virginia. Ongoing demonstrations of agricultural and domestic chores, such as spinning and candlemaking, round out this history lesson. Adults $1; children under 12, 50 cents. 703/442-7557.

ANACOSTIA NEIGHBORHOOD MUSEUM -- Friday, 10 to 6; Saturday and Sunday, 1 to 6, 2405 Martin Luther King Jr. Ave. SE. Afro- American history lectures to ages four through eight as well as group tours and teaching services. 287-3369.

NIGHT HIKE -- Friday, 8 to 9 p.m., Amphitheater, Burke Lake Park, 7315 Ox Rd. (take I-66 west to Rte. 123 south which becomes Ox Road). Take a night hike through the woods with a naturalist to discover the world of frogs, birds and other night creatures; bring a flashlight. 451-9588.

CHILDREN'S RADIO THEATER -- Saturday, 8 a.m., WPFW 89.3 FM, "Fin McCoul." An Irish legend about a giant with special powers. Listeners can participate in a call-in following each program by calling 783-3104.

CRAWLING RACE -- Saturday, 10 to noon, Reston Community Center, 2310 Colts Neck Rd., Reston, "2nd Annual Baby Crawl." Crawlers and toddlers under two years old can participate in a race across the community room floor; awards given. No established walkers may enter. Pre-register by calling 476-8192.

BATTING PRACTICE -- Saturday, 10 to 3; Sunday, 1:30 to 4:30; Tuesday and Thursday, 3 to 7:30, Tucker Road Community Park, 1771 Tucker Rd., Fort Washington, Md. Batting cage and automatic pitching machine; bats, balls and helmets provided. $1 for 20 balls; team reservations. 301/248-4404.

CARNIVAL DAYS -- Saturday and Sunday, 10 a.m. to 7 p.m., Pet Farm Park, 1228 Hunter Mill Rd., Vienna. Carnival rides, games, and live entertainment. $4, adults; $3.50, children ages two through 12. Admission includes free hayrides, tours and animal food. 759-3637.

GAMES GALORE -- Saturday, 10 to 4, Greater Laurel Nursing Home, 14200 Laurel Park Dr., Laurel (take I-95 north to Rte. 198 east, turn right at first light onto Van Dusen Street; carnival approximately two miles on left), "Second Annual Carnival." Games, prizes, food and fun for just 20 cents per game or purchase. 953-7980.

CHILDREN'S OPERA -- Saturday, 11, Wolf Trap Farm Park for the Performing Arts, 1624 Trap Rd., Vienna (take Rte. 7 west form the Capital Beltway for 21/2 miles and turn left at Towlston Road, at the brown Wolf Trap directional sign), "Island of Tulipatan." A special one-act opera for kids based on Offenbach's opera about the confused romance of a boy and girl. Free, but reservations are necessary. 255-1827.

EVENING CAROUSEL RIDE -- Saturday, noon to 6, and 7 to 11 p.m., Glen Echo Park, Glen Echo. Hours for the Glen Echo Carousel will be extended for evening fun this Saturday only; rides cost a quarter. Families encouraged to bring a picnic dinner. 492- 6282.

OLD POST OFFICE -- Saturday, noon, Pavilion, 1100 Pennsylvania Ave. NW. "Magical Mime Show." Sarah Ewing performs a show of mime and magic. 289- 4224.

TROLLEY MUSEUM -- Saturday and Sunday, noon to 5, Northwest Branch Regional Park, Bonifant Road between Layhill Road and New Hampshire Avenue, north of Wheaton. Trolley rides: $1, adults; under 18, 75 cents; under two, free. 384-9797.

CHILDREN'S PLANETARIUM -- Saturday and Sunday, 1 and 4, Rock Creek Nature Center, 5200 Glover Rd. NW. First show for children four to seven (must be accompanied by an adult); second show for ages seven and up. No latecomers are admitted. 426-6829.

ADVENTURE THEATER -- Saturday and Sunday, 1:30 and 3:30, Adventure Theater, Glen Echo Park, Glen Echo. "Toby Tyler," the story of a young boy who runs off to check out life under the big top. $3, group rates available. 320-5331 weekdays and performance days from 10 to 2.

DOLL EXHIBIT -- Sunday, 11 to 4, Sully Plantation, Sully Road, Chantilly (take U.S. 50 west to Sully Road; park entrance 3/4 mile on right). A staff member talks about late 18th- century dolls; from 1 to 3, an experienced doll maker demonstrates how to create a porcelain doll's head. $2 admission includes house tour. 437-1794.

ALL ABOARD -- Sunday, 1 to 3, Burke Lake Park, 7315 Ox Rd., Burke. (I-66 west to Rte. 123 south which becomes Ox Road). Take a 15-minute ride through the woods on a miniature train to learn about forest life. 50 cents per person (non-county residents must pay $3.50 park entrance fee). 451-9588.

CHILDREN'S GAMES -- Sunday, 3 to 4, Clara Barton National Historic Site, 5801 Oxford Rd., Glen Echo, Md. Spend an hour with a park ranger who'll show you how to invent your own games just as the kids of yesteryear did. Before and after the program, you can also take a tour of the original Red Cross house and learn about life as a 19th- century child. For children ages eight to 12. Free, but reservations necessary. 492-6245.