Here's a few things...
If you don't get a chance to do this much, get dressed up and take her out on a fancy date. Nice restaurant that SHE likes, a movie that SHE likes, and if you guys are sexually active don't worry about yourself for one night and please her.
Girls are also into jewelry so a cheap ring or bracelet or necklace or earrings all suffice. Unless she's not into that. There's also those colored rubber-banded watches with the "bedazzled" faces that girls are into.
Perfume but that shit is expensive. One year I tried to get out of a gift by telling a girl I'd get her some perfume. When I went into the store and looked at the prices... instant regret.
Another cheap thing would be to make a stocking full of candy and snacks she likes and other cool little things like an iTunes (or a CD)/restaurant/retail gift card then take some glue, write her name with glue then dump glitter on it, then beat them out. Her name will be all glittered up at the top. I did this last year for my ex.
Think of something she's mentioned wanting before and get her that.
If she plays video games, maybe get a couple of those and a couple of movies or seasons of TV shows she likes.
Hope these help.