outoftownjew
I think (and hope) that after 15 years of marriage you should know your wife well enough to know what she likes. Think about things she has mentioned and pointed out or oohed and ahhed about.
If you can afford it either buy a nice piece of jewelry, maybe theres something she really wants like a certain type of earrings or the like…
If you have the money and the time, go on a vacation with her even for a weekend, spend a few days just the 2 of you.