Hi, Katherine! I certainly understand your dilemma and concern. My mother-in-law accompanied us on a family trip several years ago and had the same worries. However, we found that there were very few attractions which she felt uncomfortable enough
not to ride. Just to give you an idea, here are a few that you may want to inquire about with the Cast Members located at the entrance to each attraction about your individual circumstance:
Magic Kingdom -- Astro Orbiter, Space Mountain, Stitch's Great Escape and The Barnstormer
Epcot -- Mission: Space
Disney's Hollywood Studios -- Rock 'n' Roller Coaster Starring Aerosmith
Animal Kingdom -- Expedition Everest, Primeval Whirl
These were the attractions that either had fixed restraints (as opposed to belts) or did not have a specifically larger seat available when my mom-in-law visited with us. Therefore, she chose to sit these out. However, what she found uncomfortable and your feelings and opinions may differ. I think that there is so much to do and see that you're not going to have any problem finding more than enough to enjoy together. I hope this helps to relieve your concerns and in your planning process. Have a wonderful day and a magical vacation!