When I was single I didn't do anything out of the ordinary: went to class, etc.
Now that I'm not, I think V-Day is dumb. I have a
post about it at my new blog.
If you're single, and you'd like to do something fun, I suggest a shopping spree. (Make sure you need those things tho. Shoes are always good, you always need shoes.)
Ask a best friend to buy you a red teddy.
Do something fun/funny with friends. Or just curl up with a good (non-romantic) book.
O... and I just thought of something else that you could do: with a bunch of friends, dress up and find a bunch of desperate single guys to pick up. Make your own version of Joey's "Valentine's Day Massacre"