Sorry, but why dont you ask her what she wants.. after all she though of things for you all these years.. maybe she just wants a quite life with family, maybe not.. depends on what kind of person SHE is...
My wife intimated that she would like to spend her 50th birthday on an exotic beach. The result was a 3 week holiday on the island of Mauritius. A bit more expensive than a normal present but I know she thoroughly enjoyed the holiday.
It depends what you're Mum's interests are and what sort of person she is. If she is an adventurous person she might want to try something new. 50 will be my next big one and it is a bit of a funny age, so do something that emphasises her youth and enthusiasm for life rather than making her feel like she really is 50! A spa day would be nice. Hope you think of something. I'm sure she will appreciate the thought you are putting into this for her.