Feeling a tad blue because you spent last weekend waiting on line at checkout counters and missed every Christmas concert in town? Wait! The season's not over yet. This weekend there's a mighty menu of musical and cultural events that will keep you hopping into the new year. And, best of all, many of them are free or just plain inexpensive. Take a look:

* The "Trees of Christmas" exhibit of a dozen imaginatively decorated spruces and firs -- the New Mexico tree is covered in red and green chili peppers -- has been extended through January 6 at the National Museum of American History. While at the Smithsonian, find out how the rest of the planet celebrates the holiday, as more than 70 musical groups, actors, puppeteers, artisans and cooks representing cultures from around the world demonstrate their ethnic customs for Hanukah, Christmas and New Year's. The celebration continues Friday through Monday, noon to 4 (the museum hours are 10 to 5:30, including New Year's Day). Free; 357-2700.

* One hundred of Greenville, South Carolina's finest young singers, dancers and actors will jam the stage of the Kennedy Center Concert Hall Friday for two performances (at 3 and 7) of the "Singing Christmas Tree." The show of secular and sacred songs sung around a 26-foot Christmas tree features scenes from a Victorian winter, a Victorian home on Christmas Eve, Santa's workshop and a living nativity. Tickets are $2.50 at the box office, 254-3600. At 6 the 60-member Youth Orchestra of Prince William presents a free holiday program in the Grand Foyer.

* Opera Bel Canto, Washington's newest opera company, will present Humperdinck's classic "Hansel and Gretel" Saturday at 8 and Sunday at 2 at the Greater Washington Jewish Community Center's Kreeger Auditorium, 6175 Montrose Road, Rockville. Tickets are $8. Call 462-1073 or 783-3370. * The Washington Cathedral's Festival Evensong will be performed by the Royal School of Church Music choir (man and boy) Friday, Saturday and Sunday at 4. The first two daily programs will be sung by a 41- voice group from which Sunday's singers will be chosen. The program of lessons and carols will span nearly 600 years of church music from Renaissance motets to modern liturgical music. Free; 537-6247.

* Saturday at 8:30 the annual Millennium Christmas concert at the Kennedy Center Concert Hall will feature the Folger Consort performing Renaissance works on the antique instruments for which the music was originally written. Then the 20th Century Consort will weigh in with works of composers George Crumb and Ottorino Respighi. The Robert Shafer Singers will also weigh in. All seats are $7.50; call Instant-Charge, 857-0900.