Valentine's Day is a day to wear your heart on your sleeve.

Here are some last-minute ideas for an impromptu party and to show that your heart is in the right place:

* Invite friends over for the evening to play hearts. Let guests rub out their scores with rubber, heart-shaped erasers (65 cents each, the Herb Cottage, Washington Cathedral, Massachusetts & Wisconsin NW).

* Cover a white table cloth with red heart stickers (5 cents each, The Written Word, 1054 31st St. NW, and 1365 Connecticut Ave. NW).

* See who can name the most songs with "heart" in the title ("I Left My Heart in San Francisco," "Heartbreak Hotel," "Peg O' My Heart" and so on).

* See who can sing them.

* Use up all the red candles left over from Christmas, adding a few heart-shaped ones so people remember what day it is.

* Serve ice cream in appropriate valentine colors like cherry-vanilla or strawberry.

* To make sure everyone enters the evening with a high heart, give out red, heart-shaped balloons ($ 1.75 for a package of six at Gifts, Unltd., 1617 Connecticut Ave. NW).

* If it's to be dinner a deux, suggest a game of after-dinner checkers, played on a heart-shaped checker board ($18 at Liberty, Georgetown Park).

* Search out the foreign-language section of a library or bookstore and find out how to say "I love you" in at least 10 languages. Say it to your guest(s).

* Try to decorate your table with appropriately named flowers --forget-me-nots, heartsease (wild pansy)--and put them in either Kosta Boda's heart-shaped bud vase ($17.50) or glass vase with three-dimensional glass heart on the side ($25; both at Ursell's, 3243 Q St. NW).

* Serve after-dinner coffee in white mugs decorated with red hearts ($ 4.95 each at the Side Door, 1365 Connecticut Ave. NW, and 1404 Wisconsin Ave. NW).

* Have someone read Robert Burns' poem, A Red, Red Rose.

* Buy a red, red rose, or a dozen. The best are expensive: $ 65 to $ 75 a dozen, or about $5.50 to $6 a stem. Whether you buy the best or rescue a bunch that's been shivering at a street-corner flower stand, slice the stems on the diagonal with a knife to make them last longer. And change the water often.

Some ideas for those who believe that Valentine's Day is best celebrated by eating your heart out:

* Suzanne's, 1735 Connecticut Ave. NW. Deliciously rich chocolate chestnut gateau will be baked in a heart shape today ($ 18), as will strawberry tarts ($ 10) and individual meringues filled with raspberry mousse ($ 2.50 each).

* Kron Chocolatier, Mazza Gallerie at Wisconsin and Western avenues, NW. Will once again feature fresh strawberries dipped in chocolate ($ 10 a box); chocolate truffles in fuschia, plastic heart ($ 30); solid chocolate heart ($ 18); half-pound sack of mint hearts ($ 10).

* Godiva chocolates, carried by Garfinckel's and Georgetown Coffee Tea & Spice, 1328 Wisconsin Ave. NW, among other stores, in special heart-shaped boxes ($ 35 to $ 40).

* Cinnamon red hots and those old heart-shaped candy mints bearing messages like Luv Ya and Be Mine.

In lieu of all that, you can always follow the lead of one sentimentalist and--guests or not--cut your toast today with a heart-shaped cookie cutter.