The pics are a great idea. A spa day is always appreciated. If you can leave the actual treatments up to her, so much the better. Another thing I would love is the promise of an evening to make me forget I'm a mom. Something really sexy and special, whatever that means for her.
I don't know how old your girls are, but something handcrafted by them is always much appreciated by every mom, I think. Or a firm date set to get a bunch of stuff checked off the "honey do" list. Or a promise of a day that she can use however she wants, whether a romantic day with you, or dragging you to the mall to watch her try on outfits, or a family day, or a day by herself, or with her best friend . . . Tickets or plans to attend something you normally wouldn't go for--a musical, play, music concert that's not your style . . .
I'm not particularly materialistic and I'm really picky and hubby is not good at figuring out my tastes, but many women do like some money spent on them. Jewelry is nice, and something simple and classic works for just about anyone.