Yes, I am in a long term relationship so a few ideas from me might sound almost comical but I've spent many a Valentines alone, not just due to being single but because sometimes you can't spend it with your beloved! Here are the things I like to do when I need a pick me up.
Get yourself a bath bomb or bubble bar and have a nice long soak, maybe a glass of wine...tub of Ben & Jerry's... you know how it is. Have some you time, indulge while reading a good book or even do a Bridget Jones and de fuzz and preen yourself to perfection. Nothing better than feeling squeaky clean.
Change your sheets, sounds more like a boring chose but there's something so satisfying about getting in to a freshly made bed, get all snuggly in some pj's, order a big pizza or some Chinese food and just chill while watching something, avoid soppy movies that will lead you to cry all over said bed sheets.
Go out with your pals, I'm sure loads of people aren't doing anything on Valentines. I've noticed so many of my friends no longer 'celebrate' the day so get your glad rags on and go for a night on the town and don't forget to belt out some Beyonce. Also it totally counts towards your cardio targets, dancing = working out so you don't even need to feel bad about eating all the pizza and not going to the gym.
Retail therapy is always a good way to treat yourself, either alone or with a friend having a look and bagging some bargains it means you avoid all the lovey dovey crap on TV. Let's not forget all the valentines day stuff is massively overpriced so who's the real winner here.