I think it really depends on her interests. For instance, we spent my birthday at Disney and we did a few things - rented a cabana all day (we stayed at the Poly), saw the Spirit of Aloha show (never had done this before but never will again), and we went parasailing! If your mom likes shows, try the Hoop Dee Doo (VERY fun and they will celebrate your bday). You could go horse back riding or do a wagon ride or other things like that at Fort Wilderness. Or you could do a super nice dinner - like Victoria and Alberts or California Grill.
I know lots of people do the 'in room' celebration but that is usually just marked up merchandise placed in your room. You could always buy stuff before you leave and put it in the room yourself.
Anyway, there is sooo much to do on any budget for any type of person. But you need to think about what your mom would like (is she adventurous? studious? a foodie? like to be pampered?). You could book a tour in one of the parks (AK trek, EP segway tour, MK behind the scenes, EP the land tour) or book some time at a spa or do something poolside, or go fishing or boating.
So you are RIGHT to say you are overwhelmed with choices! Maybe if you give a little more info on your mom and what she likes?