Just think about what she likes. And then put time into an idea that relates to that.
Example: I have always loved this series called "Percy Jackson and the Olympians" you've probably heard of it. Anyway, in the book there's this camp and for every year you go they give you a clay bead to put on this leather necklace. Each bead represents main events that happened that year. My boyfriend took this and made me a necklace with little pictures to represent our best times together. OH and just so you know, what most guys would call "gay" most girls would think it was a cute gesture.
Make her a blanket, a heart necklace on a teddy bear, a dozen roses with one fake and write I will stop loving you when the last one dies, a list of reasons why you love her, first movie you ever saw together, a book from her favorite author, if you can sing, sing her favorite song to her
maybe a picture frame with a cute pic of you guys in it. Airbrushed matching t-shirts. Of course as others mentioned chocolates and roses. But I think something thoughtful and creative is the best thing a boyfriend can do. Take her on a cute little picnic..
Gift her something that she remembers you for a lifetime, if you are really serious about this relationship. As there are many options available to buy anything precious through online deal, where you can possibly get good deal within your price range.