THE DAY AFTER Christmas was fine. They played with their toys, rode their new bikes, listened to their Walkmans, slathered their sticky candy canes over the furniture.

But now the big letdown has set in. And it's time to get them out of the house, preferably to some place the post-Christmas crowds are not. Here are a week's worth of ideas to ease the transition from the Christmas high to the back-to-school doldrums.

SCAVENGING AROUND -- Kids are natural scavengers, so they'll enjoy a natural scavenger hunt. They can join one Friday at 2 at Hidden Pond Nature Center in Springfield, led by Elianne Lieberman. Phone 451-9588.

If you can't make that, organize your own hunt in your backyard, neighborhood or park. Naturalist Lieberman suggests making your list general, so there can be more than one right answer. For example, ask the kids to find something rough -- which could be some bark, a rock, or a lichen. Or ask them to find a leaf with pointed edges, rather than specifically for an oak leaf. To protect the environment, items should be checked off or described rather than actually collected.

GO SEE IT ON THE MOUNTAIN -- This weekend's your last opportunity to catch the Adventure Theater production of "Heidi," the classic about an orphan from the Alps who helps cure a poor-little-rich-girl from the city. An outing to the theater in Glen Echo might be equally therapeutic for your children. Call 320-5331 for ticket information. The show runs through Sunday.

While you're at Glen Echo, lead the troops across the parking lot to the Clara Barton House (492-6246), open daily 10 to 5 for tours. Barton built the house from lumber from four temporary hotels the Red Cross had put up to shelter victims of the Johnstown flood. The big yellow hous as her home and as a Red Cross office, warehouse and boarding house for volunteers. Kids like the bigness of the house and looking into the closet where they find such things as a 19th century version of the Band-Aid, says Park Ranger Elizabeth Singleton,

If the kids still have energy to burn, let them hike it off on the C&O Canal towpath, just a quarter-mile down the road.

BUSY AS A . . . -- Beavers don't have time for post-Christmas depression because they're too busy gnawing down trees and building dams. There's an active beaver site in Prince William Forest Park in Triangle, Virginia, and at 1 on Sunday a park ranger will lead a hike to the beavers' lodge. If you're lucky, you'll see some of the critters. In any case, you'll hear a talk about one of nature's best conservationists. Phone 703/221-7181.

CAPTURE A UNICORN -- Or a lion, or a knight in shining armor. The London Brass Rubbing Center, in the crypt of Washington Cathedral, has these and other images in reproductions of brass reliefs from English churches. For $2.25 to $20 -- depending on the brass plate chosen -- you can rub an image onto paper.

The beauty of brass rubbing is that the finished product almost always looks good enough to frame. This activity is suitable for children six and older, and there will be plenty of staff on hand to help out during the school break. The fee includes all supplies and instruction. The center will be open daily from 9:30 to 5. Phone 364-9303.

SEND IN THE CLOWNS -- Better still, take your children to see the clowns and lions and horses and tightrope walkers of the Howard Bros. Circus at the National Geographic Society's Explorers Hall, open Monday through Saturday 9 to 5, Sunday 10 to 5.

The circus is only one-sixteenth life size, but there are a million parts to it, all under old-fashioned big tops, or little tops. The mini-circus is the work of Howard Tibbals, who got hooked on circuses as a kid and froze it in time, circa 1930. It includes a rare look at the circus back lot, where off-duty clowns are relaxing in their underwear. You follow the crowds to the side shows and the menagerie and into the big top, where horses prance and tigers threaten and trapeze artists risk their necks. No detail is overlooked. You can even see the workers preparing food for the zebra. Phone 857-7588.

While you're in the mini-mode, take your little ones to the Washington Doll's House and Toy Museum where dozens of doll houses have been decorated for Christmas. There are also life-size antique toys and games and a rare antique Christmas tree made of dyed turkey feathers. The museum is open Tuesday through Saturday 10 to 5 and Sunday noon to 5. Admission is $2 for adults, $1 for children under 14. Phone 244-0024. It's at 5236 44th St. NW, handy if you need to take gifts back to Lord & Taylor, Woodies or Mazza Gallerie.

HORSING AROUND -- Winter's a great time to ride through the woods of Rock Creek Park -- not in a one-horse open sleigh but right on the horse. Rock Creek Horse Center (362-0117) is open Tuesday through Sunday for pony rides, trail rides and lessons. Guided one-hour rides are available for riders 10 and older ($9). For younger children, you can rent a pony to lead around the ring with your child in the saddle ($5 per half-hour). Lessons are available by appointment. The stable opens at 9:30; the last trail ride leaves at 4:15.

To inspire your children to aspire to greater riding skills, take them to a "schooling" horse show on Sunday at Frying Pan Park in Herndon. A "schooling" show is designed to introduce horses and riders to show competition. The show begins at 8 a.m. in the park's Indoor Activity Center. Take I-66 west to U.S. 50 west. Turn right on Centreville Road in Chantilly and right on West Ox Road to the park. Call 437-9101 for more information.

PUT YOUR KIDS ON ICE -- If Santa Claus didn't bring ice skates, you can rent them at most rinks. The National Sculpture Garden Ice Rink on the Mall (between the National Gallery and the Museum of Natural History) is open daily from 9 a.m. to 11:30 p.m., with a special midnight session from 11:45 to 1. Admission is $3 per session for adults, $2.25 for children under 13, who must bring an adult. Skate rentals are $1.50. The midnight session is $1.50 for all ages.

Pershing Park rink (14th and Pennsylvania NW) is open daily from 9 to 9, and Fridays and Saturdays until 11. Admission is $3 per session for adults, $2.25 for children and $1.50 for skate rental. If you want to entertain all the kids in the neighborhood, you can rent the whole rink for $75 per hour.

Herbert Wells Ice Rink, at 5211 Calvert Road in College Park, will be open 11:30 to 1:30, 2 to 4 and 7 to 9 through December 30; 11 to 2 on December 31; 2 to 4, 4:30 to 6:30 and 7 to 9 on January 1 and 12 to 2 on January 2. Admission is $2.50 for adults, $1.50 for children 12 and under (who must be with someone 16 or older). Skate rental's $1.50.

Mount Vernon Recreation Center, at 2017 Belle View Boulevard in Alexandria, will be open for ice skating this Friday from 4:45 to 6:15 and 8:15 to 10:15. On Saturday, sessions are from 1 to 3, 3:30 to 6:30 and 10:15 to 11:45. Sunday sessions run from 2:30 to 4:30 and 6:30 to 8. On Monday, there's skating from 2:15 to 5:15. On New Year's Eve, you can skate from 8:45 to 10:45 and 12:30 to 5. And on New Year's Day, you can skate from noon to 6. Admission ranges from $2.75 to $3.50 for adults and from $2 to $2.50 for children, depending on the length of the session; skate rentals are $1.25. Children under 6 must be accompanied by an adult.

If it turns really cold and the ice on the C&O Canal gets four or more inches thick, you can bundle everybody up and show them what skating was like when you were a kid. Call 299-3613 for an ice report.

FEED IT TO THE DUCKS -- In case you were wondering who was going to eat Aunt Bertha's fruitcake, this might be a good time to visit Constitution Gardens, a popular way station for migrating waterfowl just west of the Washington monument.

"They're wildlife -- we don't encourage people to feed them," says a Park Service spokesman.

But, after all, Christmas comes only once a year, and they might actually like Aunt Bertha's fruitcake. While you're in Constitution Gardens, you can take a look at Washington's newest monument, a memorial to the 56 signers of the Constitution. All 56 names are engraved on a simple monument at the water's edge.

SAYING GOODBYE TO '85 -- After all that togetherness, you and the kids may want to go your separate ways for a few hours. To that end, the Alexandria YWCA is hosting a sleepover for boys and girls 5 to 12. The party begins at 8 on New Year's Eve and concludes at 9 a.m. on January 1. Festivities include snacks, breakfast, games, hats, noisemakers, arts and crafts, movies and swimming. Call 549-0850 for reservations.