I love this question! I'm single and Valentine's Day ISN'T just for people who are in love! While I enjoy watching the people I know do romantic things for each other during this day I don't exactly envy them. People feel obligated to buy expensive gifts like diamonds and things they can't afford. I love being single!
I'm going to do for myself what I would do for a love interest if I had one right now. I'm going to take myself out to a movie or rent one I really want to see and snuggle up on the couch to watch it with a bowl of popcorn. I'm going to make myself a beautiful meal, get some flowers for the table, a nice bottle of wine or sparkling cider, light a candle and enjoy rack of lamb or crab legs and a salad.
Just because you aren't in love right now doesn't mean you aren't just as special as someone who is! If you have any friends that are in the same boat you are maybe you could have a dinner together with each contributing their favorite dish or desert.
Or if you aren't into cooking maybe treating yourself to a massage would be a nice change. You have to love yourself before you can love someone else anyway so its a great day to make that resolution to yourself.