Some notes on Switzerland:
Not being in a wheelchair and never having to travel around with someone in a wheelchair, I might be more optimistic than realistic, but I think there are some legal obligations for things to be wheelchair accessible. The range of these obligations, I am not sure, but public transportation should not be an issue (you might need to be a little early at train stations and ask the help of the station crew to board though - they have a mobile wheelchair lift for some older train models that are not wheelchair accessible - and as far as buses and trams are concerned you might at worst need to wait a little as in some area only one out of two or three vehicles on each line are wheelchair accessible) and most if not all museums and theaters should be accessible.
Now a nice part to stroll around in most cities is the old town, but for medieval defense reasons, those are often on hills... a notable and worth visiting exception is Bern (surrounded on three sides by a river). I don't know Zurich and Basel nearly enough to comment (Soriak should be able to help), but Geneva (outside of the old town) is a pretty flat city, so moving around should be easy.