Chocolate is the old stand by, but with a little creativity, you can earn many more points with your girl by getting her something tasty and exotic she wouldn't usually buy for herself then preparing it for her. The pre-made dinner aisle at the grocery store has many easy and diverse choices of foods that you can heat, serve and enjoy with her. You don't have to be a gourmet chef. Just the time and effort you put into it will make it special to her.
You could always give her a day at the spa but, better yet, why not surprise her with tickets to something you've never done before together. Bungee jumping? Salsa lessons? It may be going to her favorite concert or something as simple as playing frisbee together. Showing that you want to do things with her, whether helping her with homework or taking her for a midnight walk, can be the most romantic thing you can do.
Roses and diamonds never hurt, but why not shake things up a bit? How about a heart-shaped pendant on a gold necklace? Better yet, make something! Take a turn around the local craft store and let your imagination roam. How about wheedling her mom into giving you an old photo of her as a baby then having it blown up and framed? Or a CD of her favorite music, along with a journal with sweet notes in it from you? As with everything else, anything you put time and effort into she will treasure forever.