Soon to be girlfriend, Bday present?

So this girls birthday is next month (March) an was wondering what i could possibly get her (under $100). Im going to ask her out on her birthday as well. So i was wondering maybe i should get one of those interlocking necklaces that have stuff on them? If you could come up with any suggestions that would be of great help! (birthstone is aquamarine if that helps). I've looked yet i always go over the top with presents.