Michael,
Smart move! Show mom she is appreciated!
While I have favorites within the spas I list here, the one closest to your resort will be your best choice. Travel to the spa can make, or ruin, the entire experience.
Grand Floridian Spa
Best for Magic Kingdom area resorts.
Saratoga Springs Spa
Often rated as the best spa at Walt Disney World, this is best for Downtown Disney area resort.
Mandara Spa (at Dolphin Resort)
Perhaps my favorite, this is ideal if you are located at an Epcot area resort.
If you don't have easy access to one of these, then go with the Saratoga location. Either get her a cab to the resort, or send her on the resort bus to Downtown Disney. From there she can take the boat launch to Saratoga. Just make sure you schedule ample travel time.
As to what, always go with a standard massage, 60 minutes or more. If you can afford to add a manicure/pedicure, that is a real plus! Consider giving her some post-spa pool time while you guys take care of yourselves.
If you need more suggestions, ask me directly from my bio page!
Good son! Have a magical vacation!