If you can afford it (or talk someone else into paying for it <--- ideal), the brunch at Chateau Montebello is unbelievable---although I suspect for Mother's Day, you'd probably need to make reservations a month or more in advance. Some other ideas:
- Memory foam pillow(s) with zillion thread count pillowcases
- One of those bathrobes that are velour on the outside, plush terry on the inside
- Spa day (or 1/2 day)
- Not sure if you're a picture person, but a scrapbook of your own life is a fun idea
- Is she a coffee person? Get her a pound of really great coffee