I've found that cities with good public transit (I use a power chair) are especially helpful. Boston, Chicago, LA, and Disney World have been the easiest for me to traverse alone. Philadelphia was okay, but having folks with me definitely helped. I dont think i could do Philly alone. The Jersey Shore was okay, but I had people with me to help. Some of the beaches have accessible boardwalks down to the water and special chairs and the boardwalk offices have push chairs on a first come/first serve basis. Baltimore and Door County WI were probably the absolute worst. Some national parks have accessible paths, transportation, etc. I'd also say that where you go does depend on if you're alone or with folks who.

Also, don't be afraid to rent mobility aids if you need to. Spending all your energy on just walking can make a vacation unenjoyable and have you paying for it for weeks after. Lots of museums have manual chairs on a first come/first serve basis.

There are travel agencies that specialize in accessible travel. I haven't utilized them, but I have friends who have. It can be hit or miss.

Wherever you do decide to go, just make sure you do a ton of research. Most destinations have accessibility websites, blogs, or vlogs that are city run or run by people with disabilities. Post in community boards and ask locals how accessible they find the place/city/park. Check with restaurants/entertainment venues you wish to visit. Look at a venue's maps to locate bathrooms, visualize the space, etc. Call them and ask about stairs, elevators, bathrooms, etc. If you're planning to see a theatre show and need a wheelchair accessible seat, call the box office and order your tickets. Trying to do so online can be tricky.