Boston has lots of cobblestones, which may or may not be easy for a scooter, depending on the kind of scooter. I have heard complaints about wheelchairs not being very good over the cobble, but on the other hand, a motor scooter will be much easier. On the other hand, in Boston, you can also take tour buses that are very good and give you a very good idea of the city, and which don't require walking (they're water/land tour buses that also go out in the bay, very cool). On the OTHER other hand, many things in Boston that are historical (don't know if you care about US history, but if you do) are, well, historical, so they're kind of tiny and hard to get into, certainly not very accessible - places like Paul Revere's house, etc. On the OTHER other hand, there's also lots of other places that are accessible, like the Boston Museum of Fine Arts. I'm not sure how well the Isabella Gardiner museum does but if you decide to go to Boston I live here and I'd look into it for you. It's a can't-miss if you're here. There are some cons here but I don't know about their accessibility.

Las Vegas is generally very accessible, IIRC.

Yosemite National Park is, IIRC, fairly accessible and extremely beautiful, but you may find it frustrating that some of the most beautiful views will be impossible for you (due to, well, you can't really hike up Bridal Veil Falls or what have you). That said, you can rent a scooter there and apparently get some kind of accessibility pass that will let you drive on some otherwise-private roads to get to more things. I'm not sure what kind of proof of disability you'd need to get that pass, if your disability is invisible, but I'm sure it's possible. If you go to the Yosemite website there's an accessibility guide and I bet it would help a lot if you get to that stage of the planning.

Good luck!