Roses would be lovely. You pretty much cannot go wrong with flowers. Unless she has allergies or something.

If you think something else would be more meaningful, try jewelry. Or an IOU for a romantic dinner out. Or a gift card to her favorite store. Or a bottle of her favorite wine. Or a promise to do some dreaded house chore that she routinely takes care of for a week. Vouchers for domestic assistance of some kind are very sexy gifts in our book. I expect she'll appreciate any gesture. If it were me personally, I'd be very impressed with roses or some other flowers because the other people who're standing around will see those, too, which is always a nice public display of affection.