I would recommend perhaps a commemorative ring or necklace. Another cool thing you could give them is a photo album of the past year, or even make a video collage of the year you spent together.
As with any gift it should really be more about the thought behind it than the price tag. I have found that the best gifts are the ones that have a meaning behind them that only the two of you know or understand. Making a mixed cd of all their favorite songs is always a good basic choice.
A gift certificate for dinner and a night at a nice hotel assuming that we're talking about wedding anniversary. For a young couple who are not married, a scavenger hunt revisiting those places that were memorable firsts is a romantic way to spend the day with small gifts at each place. Well worth the preplanning.
Jewelry makes a nice gift, just don't do rings yet. You might consider re-creating your first date, except add flowers and/or candy. A gift certificate she can use toward a favorite hobby or interest would be good too. Try to do something that shows you know her.
A good choice would be anything that shows you really know your bf/gf as a person. For example, if your girlfriend likes penguins, something penguin themed shows you care.