Let me put a few ideas in your head before you head out to the mall. You can do the flowers thing, nothing wrong with that, but lets not forget that the day can be a whole experience. What does that mean? Don't just show up with chocolates and flowers, make it special. Groom yourself a little more than usual and put on some cologne (just a little). Buy some classic music, Sinatra and or Johnny Mathis are great choices.
Valentine's Day is your chance to take some risks so don't be afraid to introduce something spicy to the relationship. Maybe you have never played with toys or watched porn together, now's your chance. If these ideas are shot down you can chalk it up to you trying to be spontaneous and adventurous. It's like a free play on a defensive penalty! Of course you can't go too far overboard, but if you want to end up in that direction eventually, V Day is your way in. Let's say you want to get into kinky sex, then start with a few hints and suggestions tonight. Introduce the ideas and let her open the door further.