Trust me... you cannot go wrong with the spa thing because they get pampered all day and love it. It's usually what I get chicks for Valentines Day. It never fails- they come home and
Just make sure you pick a package and don't just get a $ gift certificate. Then you're forcing her to pick what services she wants and she may want more than you spent. I've found that a lot of spa's have packages that include whatever but some have just regular old gift certificates. Stay away from regular gift certificates. You want it to be more personal than that.
P.S. Spa's/Massage parlors on the side of the freeway should be avoided, lol.