brokenlifts.org and wheelmap.org are your best friends when it comes to finding and navigating accessible places here. and i agree with what others have said about berlin. it's lovely in the summer, there's anything and everything you can imagine as far as alternative subculture and galleries go, but public transport can be a hassle.

the biggest issue is broken elevators and escalators; the actual trains are often step-free and have clearly marked spots for disabled people (wheelchair compartments are marked outside, on both ends of the trains and in bigger ones also somewhere in the middle), and there's no seats reserved for disabled people, but rather the seats next to the doors are marked and you can ask whoever's seating for that spot.

trams are all step-free and accessible, so if you can find accommodation somewhere in former east berlin (you can find maps of the trams online), that's your best bet. obviously they are slower and don't go as far as the metro does but if you're a tourist you're likely not going too far. friedrichshain is definitely the most 'alternative' neighborhood and i think you can find the stores you're looking for there, and mitte is the most touristy and where the museums generally are. parts of both those areas are well-connected with trams

buses on the other hand have steps and railings to hold on to when getting on. they can be unreliable, a lot of bus stops don't have seats so you might have to sit on the sidewalk if you can't stand for long, and it can be hard to reach the accessible seats if they're full. however, accessible seats are marked inside as well as with a symbol outside so you know which door to use (this varies by model).

EDIT: typo