GF and I of only a few months are both pretty open about this kind of thing, so we put a $200 limit in place due to the short amount of time. I figured that was pretty appropriate, given a dinner date up here can easily eclipse $100 even for something casual.
I got her a Michael Kors fashion watch from Amazon and some crystal earrings off of Gilt. Hopefully she'll approve of my choices. She really needs a watch.