Flowers? That's pretty presumptive and sexist. Gift card? Better not be to Bed, Bath and Beyond or something like that.

The PC way to go about it is to entirely ignore the day because you presumably pay your assistant what they're worth, just like any other employee at the office. No preferential treatment should be allowed, really, because everyone is basically the same regardless of race/gender/sexual preference/height/weight.

(I think I did that right, if not I'm a straight white man so just blame me for whatever, that always works)