Which places in Europe are easiest for people with limited mobility?

The answer is, "it depends". If you're too old or infirm to walk, how do you get around? If by wheelchair or bike you'll have options, but electric scooters are not common. Most train stations have some accommodations for wheelchairs, but that's in the big cities. Ramps are not common. And many roads and pedestrian areas are cobblestone which is not easy to traverse. If you can maneuver on your own, with or without canes, but just not far, then you can plan to use the trains, trams, subways, and taxis'. Elevators are rare outside of modern hotels and stores, and public buildings. Likewise escalators.

Let's say not people in wheelchairs, but rather people who walk but relatively slowly and not too far, maybe with a cane. Which places or sorts of places are most amenable?

Hop on hop off tourist buses in mostly flat cities like Madrid, London, Paris, Barcelona, Vienna, Berlin

Amsterdam has flat terrain and its network of trams is a big help for those with limited mobility. The biggest thing you would have to watch is bikes, i.e. being able to get out of the way fast when crossing bike routes.

Yes Amsterdam but not solo.

Sweden has good public transportation and the bus driver will lower the bus on public transportation. With a wheelchair, there are ramps for trams. This is generally true in larger cities.
I am not one for making sweeping generalizations about either Europe or Sweden, where I live, because accessibility varies even within a country.

Let's say not people in wheelchairs, but rather people who walk but relatively slowly and not too far, maybe with a cane. Which places or sorts of places are most amenable?

As someone who has needed to use a cane since before the turn of the century, it still depends. Mobility impairment is a sliding scale, and even in the same person can vary greatly from day to day. And what I am capable of doing may bear no resemblance to what you can do.

Aside from climbing mountains or adventure hiking, there aren't many places I've avoided solely because of my disability. The key is adequate research. Know your options in any given place and plan accordingly. I cant climb the stairs leading to Sacre Coeur, but i CAN take the funicular. I cant walk without a break all the eay from the Arc de Triomphe to the Louvre, but I CAN do it by resting once or twice on a bench in the Tuilleries. If a city has a metro system, do the stations have reliable escalators/elevators? Is the bus/tram system coverage adequate for the places I want to see? How expensive are taxis or ride sharing? At sightseeing places of interest, read their websites before leaving home. Most will have some accessibility information.

Travel outside large cities means reduced or practically no public transportation; but that may mean much easier driving and parking. Hotels and Inns may not have elevators, but you can request rooms on the ground or second floor. There may be a 1 mile walk to a scenic lookout that is uphill (both ways). I can't do that as fast or as easily as DH could on his own. But I can still do it at my own pace, stopping for a minute if I need to. Above all, know your own limitations and don't delude yourself into thinking you can do something that is physically beyond your capabilities. Otherwise, at the end of the day, you may find yourself complaining about how hard everything is and how awful your day was.

As orientation: the ENAT provides a list of annual winners of the Access City Award.

Personal experience of my uncle, user of a walker: Northern German cities with heavy damages from WWII needed to be rebuilt, so they do not or less have old copperstone pavements/ okd towns and their ground level is mostly even (flat land). Marathon in Berlin for example is known for its very limited height difference. If someone wants to visit a certain city hopefully information about accessibility is provided by the destination (example Berlin). The large area of these cities can be bridged well by widely accessible and cheap public transport.

Germany's largest train provider Deutsche Bahn offers a mobility service - free of charge (flyer).

I walk a lot. I walk more in the really large cities because there are so many sights, and they aren't concentrated in a small area. Surface transportation would help a lot, but I use it only in extreme cases. If the traveller can't easily board a bus, that would mean taxis and could get expensive.

My first thought was of smaller cities that happen to be relatively flat. Padua is one good choice, because most of the sights are within a rather short walk of the tram line. Ravenna's flat, but a person might want to use a taxi from the train station to a hotel selected for its location sort of in the middle of the mosaic sites.

I think it might be easier to compile a list of popular destinations that would be unusually challenging, because with advance research, one can manage in a lot of places as long as one has reasonable expectations about how much territory and what specific sights can be covered in a day. I regularly delete interesting-sounding villages from my itineraries because they are too challenging or costly to reach without a taxi or rental car. The mobility-impaired can attack city sightseeing the same way if they're OK with focusing on what they can do rather than regretting what they cannot. It's a question of how much one wants to see the tough-to-reach places--i.e., how much money (or occasionally time, in the case of rural buses) one is prepared to throw at the problem.

Venice would go on my list of very difficult destinations because of the need to cross so many little bridges. Rome's challenging because of the hills that pop up everywhere, broken-up pavement in some areas and wide distribution of sight most people want to see. And Pompeii without a wheelchair would be a definite "no" for some folks because of the giant cobblestones and elevated sidewalks.

This website is a great resource for tips on accessible travel destinations in Europe:
sagetraveling.com

Check RS helpful tips for travelers-with-disabilities

Any location is going to have issues. I would select a place you are interested in, then work with a guide or local representative to map out your days such that you enjoy them.

Interesting perspectives thanks!

OP if you find this useful, here's what I wrote about my trip back to England with my own mobility issues -- https://community.ricksteves.com/travel-forum/trip-reports/my-journey-back-to-englad

I've gotten a lot stronger since last October, back then I thought London was easy but my attitude has always been, "suck it up, let's do this."

VAP how did the car hit you? Were.you on a bike?

I don't know how I was hit, I have no memory of it. But I was on my bicycle when it was struck by a car at 50mph.

Like I said I found London easy. What was worrying me before the trip was being in the way of other people especially on the Tube with a non-flexing knee, but I was met with nothing but kindness.

Glad you lived and that travel is still in the mix for you. Thanks for the feedback on mobility and travel.

I am an architect, so designing for compliance with the Americans with Disabilities Act is a way of life. I am not a great world traveler as many of you are, but in the traveling I have done, I have yet to find a city or country that compares to the US in terms of accessibility. So, unless someone can point out exceptions, do not expect the same ease of movement that you enjoy in the US.

If some one needed assistance I could map out a tour in my home city in Europe. The challenges, even for someone with limited walking abilities, in my city and many cities in Europe will be entering and exiting public transportation, each and every entry door step, avoiding cobblestone streets, and knowing who has restrooms that are not located "down stairs".

Europe is improving their sidewalks and stop lights to where they're more handicap friendly. Most major cities you can get by on buses and trains and they're getting better all the time.

My wife can walk but her distance is very limited. She travels on a folding TSA approved electric personal scooter. So far, it's been to Europe 3 times, to Las Vegas and to 3 National Parks. It's simply a Godsend with a 10+ mile range and it's faster than I can walk. She rides it up to the door of the airplane.

We found some newer Tube stations in London to have elevators and escalators. But many of the old stations had a bunch of steps. Venice is a place to avoid with all the arch bridges going over the canals. We have found buses just about everywhere to be handicap oriented. We now stay in more modern hotels with elevators--rather than in B&B's with stairs. I would avoid Lisbon and places like Cinque Terre and the Amalfi Coast of Italy. We are now mixing in a cruise in the middle of our trips--and they're very good for mobility challenged folks. At one point, you might want to look into guided bus tours.

I'm asking out of curiosity, not immediate needs. What are good destinations/itineraries/tips for tourists who don't walk quickly or far

You’d really need to consider what type of thing you’d like to do and see. Here in Switzerland we are in the final stages of making all train stations and rolling stock were accessible for people with physical handicaps. So if you wanted to ride the famous tourist trains you’d be fine. On the other hand if you wanted to visit old towns and so on it might be limited due to a lack of public transport and so on.

My husband can’t walk quickly or far. We have not picked any destination based on that. What we do is to stay as close to the major sights as possible. We walk about 10 minutes and look for a bench or a church to sit and rest a while, then continue. Also, if he’s too tired taxis or Uber or Bolt can be used. In the past we walked or took public transportation where available but now health is a factor so we use rides when they make sense. Our day starts later and ends earlier. Also, lunches have gotten longer as need for rest time has grown. We start with an itinerary of things to see and do and then cut down the list.
So far we’ve been able to adapt and enjoy the experience of different cultures. I hope that you c, too.