Send her flowers. It may seem cliché, but nothing says "I love you" like a beautiful bunch of flowers. However, you should try and put a little effort into it - don't just pick up the first bunch that's on sale at the supermarket. Go the extra mile by finding out from friends or family what type of flowers are her favorite and have them specially delivered to her home.
For added romance, you can attach a romantic note to the flowers. It can be something short and sweet like "beautiful flowers for a beautiful girl" or something a little lengthier, like a romantic quote from a poem or song.
If you're short on cash, don't worry. You don't need to buy the most expensive, imported flowers available. In fact, a single red rose or a freshly-picked bunch of wildflowers can be infinitely more romantic than a generic, pre-arranged bouquet.