THE LATE-DECEMBER BLAHS. Mary Johnson of the Kennedy Center diagnoses them this way: "The holidays are over; everyone's crestfallen, and what do you do with these beasts?"

This weekend, there's quite a bit to do with beasts and their parents (or beasts and their children, depending on what kind of holiday you had). Out of the nearly six billion available activities -- everything from playing Clue with Uncle Norman to laying down Big Bucks for a night at the ballet -- we've zeroed in on the following freebies (or near freebies) for families to beat the blahs.

These are the ones that, in our opinionated opinion, are worth braving the holiday crowds for:

AN AFTERNOON AT THE KENNEDY CENTER: If you get to the center around noon any day this weekend, you'll be entertained (for free) by singers, dancers, instrumentalists and a juggler in the Grand Foyer. And for $2.50 on Friday, you can watch a Christmas tree come apart.

The "Singing Christmas Tree" is an 18-year tradition in Greenville, South Carolina, which sends north a hundred of the town's "best young singers" to mount a 26-foot-tall scaffold (built by the local Rotary Club) and sing. They dismount to dance, act, form a Nativity scene, and sing the Hallelujah Chorus, Hanukah songs and Christmas pop for "anyone who can sit still for an hour," says Johnson.

Everyone who enters the Concert Hall must pay for a ticket, so you might leave the baby behind in the company of any ultra-cool teenagers in your family. But anyone with a "shred of sentimentality" should love the show, says Johnson. Performances are at 3 and 7 p.m.

A MORNING AT THE NATIONAL: Anyone who remembers what it was like to be a good little girl or a tough little boy when babies were booming will appreciate the idea Marlo Thomas presented in her early '70s book and film Free to Be You and Me -- that it's okay to be ourselves without regard to stereotypes about our gender, age or race.

As part of the continuing Saturday morning series at the National Theater, Creative Company this Saturday will present poetry, drama, song and dance based on selections from Thomas' book by people like Judy Blume, Shel Silverstein and Gloria Steinem. There are performances at 9:30 and 11:30 a.m.; admission is free, but reservations are required. 783-3370.

A HALF-DAY AT THE HOLIDAY CELEBRATION: It will probably take you at least half a day to wade through the Museum of American History's annual celebration, with 70 different singing groups, musical ensembles, craftsmen, cooks, and performers putting forth from noon until four through Monday. Especially fun for families with children are the German and English precursors of Punch and Judy (on the third floor, East side, starting at 1:30), and the two storytellers in the quilt corner on the first floor, West side (starting at 12:30).

There's also food -- Sephardic spinach meatballs on Friday, Polish pierogi on Saturday and Swedish Lucia buns on Sunday -- demonstrated on the first floor, East side. And family members can try their hand at crafts like paper cutting, and Moravian star or dreidl making (various locations).

Actually, half a day may not be enough time.

A SATURDAY STROLL: Two parks offer an opportunity to walk off that Christmas cholesterol this Saturday. Those with scouts in the family might enjoy toting their compasses down to Prince William Park, near Quantico, for a "land navigation hike" at 1 p.m. It'll help people bone up on their orienteering skills. (call 703/221-2104 for directions).

And anyone who longs for a pleasant walk along the C&O Canal should enjoy the one scheduled for 10 a.m. at Great Falls Park in Maryland (call 299-3614), where a ranger will stroll along and point out the "sights and sounds of winter."

A DAY AT THE ZOO: Any winter day is a good time to hang out at the zoo -- the tourists are gone -- but two special events make this Sunday a guaranteed hit. Bob Brown Marionettes will perform at 1 outside the Education Building (the one closest to Connecticut Avenue). Those who get to the building early (like when it opens, at 9) and remember to bring a one-foot-square cloth can sign up for a puppet-making workshop the company will hold after the performance.

If you're not among the 45 able to sign up for the workshop -- or even if you are -- here are some other fun things to see: 11 and 3, Panda feeding; 11:30, Seal and sea lion training; noon to 3, Zoolab (Education Building), Birdlab (Bird House) and Herplab (Reptile House); 2, Elephant feeding and training.

A MOMENT WITH MINIATURES: Over at the National Geographic's Explorers Hall (17th & M Streets NW),they have 24 different miniature scenes ranging from a bakery, a greenhouse and an eight-inch, fully operative grandfather clock (wound every two days with a gold key) to a Pacific Northwest Gothic-style dollhouse, complete with growing moss. The exhibit includes a film with miniature craftsman William Robertson telling how all of this is done. Admission is free; the show is open Monday through Saturday from 9 to 5, and on Sundays from 10 to 5.

A WEEKEND AT HOME: If you don't think you can bear one more trip out, try celebrating at home. Author Carol Brink would have been 89 on Friday, so this might be the perfect weekend to curl up with the kids and read her classic pioneer story, Caddie Woodlwan (MacMillan, $2.95, also available in area libraries). And hide out until after New Year's.

Deborah Churchman last wrote for Weekend on Rose Hill Manor.