Suppose you don't want to break it up? If you're happy with what you've got, cement the relationship with a Valentine's Day present. You don't have time to buy anything mongrammed, but here are some gifts you can get before the deadline: ONE PERFECT ANTHURIUM, a big, bright-red, heart-shaped tropical flower flown in from Hawaii, costs between $4 and $5 (depending on the weather in Hawaii at take-off time) at Flower Inc., 1431 H Street NW. For a fee, they'll even deliver. Call 737-3232. A HEART OF GOLD, 24 karat gold, will tell exactly how much you care. A one-gram heart will run you $75. A three-gram heart will set you back $150. If you're really big-hearted, go for the five-gram heart at $225. At Lord & Taylor, Fair Oaks, Chevy Chase, White Flint and Falls Church. SOCK IT TO HIM with a pair of red-and-white, heart-adorned hose. They're $3.50 at Kay's Men's Shop, 1020 15th Street NW. FLOATING ON AIR? A heart-shaped "puffy," helium-filled mylar balloon, is $2 at The Kite Site, 3101 M Street NW. Call ahead (965-4230) for more than one. SAY IT WITH MUSIC. Better yet, let the Thomas Circle Singers phone it for you, lyrically. They'll sing the song of your choice for $10. Long-distance charges, if any, will be billed to your phone number. Call 338-8816 to order your song.

LOVESICK? A heart-shaped sterling pillbox will carry the cure nicely. The Tiny Jewelbox, 1143 Connecticut Avenue NW, offers a variety for $90 to $120. THAT's THE TICKET: Get two of them ($5 each) to the most romantic show in town, the Trinity Players' production of Stephen Sondheim's "A Little Night Music." It opens this Friday and playes weekends through February 28. Box office: 965-4680.