trouble walking

That question is so broad and vague that it is impossible to answer. Think if an European as you that same question about the US. How would you respond? Most European cities were laid out for walking since that is all they were doing 500+ years ago. Most major cities require an enormous amount of walking because they are big. The small towns would obviously require less but then the local transportation. I think you will need to reply on a lot of taxis to take you from one point to another. We need a much clearer idea of the travel plans before anyone can advice anything.

Most of Europe does NOT have the same accesibility laws that the U>S has., but Florence, Italy is an easily walkable city. It is flat, and the piazzas are close together... enough so that you can walk shorter distances and sit and rest between sites. Rome is also more flat in the Vatican area, and Piazzas, but the Collisseum area is very hilly, but there are many taxis. I hope you get more advice than mine, I do not use oxygen, but have a Very Bad Back, and need to stop and rest. Elevators can be scarce if a wheel chair is used. When making hotel reservations, make sure that there is an elevator (I do) and remember that a "first floor" in Europe is usually up a few stairs! Venice has only stairs when you arrive at the train station, no ramp (at least that's the way it was a few years ago) so, I'd avoid that city completely if you have a wheel chair.

Michelle, I am a little sorry about my previous reply, in that it wasn't more encouraging. This trip IS possible if you plan correctly! Make sure when you make hotel reservations that the hotel has an elevator! First and foremost! Surprisingly, many hotels in the lower to mid price range are often "walk ups!" I've encountered this a number of times, and I'm NOT YET in a wheel chair, but I DO have issues! Secondly, a busy city, i.e., Florence, Italy, Rome, Italy, Paris, France will be more accommodating than the smaller cities! I've been to Italy a lot more than the other countries, but you may also want to consider Paris. We took a lovely and inexpensive ride down the Seine River twice as our mode of transportation while there (saved on the walking) and it worked very much like a "hop on hop off" bus tour in any other city, and was especially beautiful. Just be prepared to take a taxi when needed instead of the metro/bus transportations, because many, many of them do not have elevators, escalators, etc. The price of the taxis is well worth going vs. not going and having this wonderful experience.

Michelle, How wonderful of you plan this trip. FYI, there are websites that specialize in travel advice for those with mobility challenges. In Italy, Florence might be a good idea, since it's flat, with lots of squares and cafes to rest in... but it's also crowded. Many historic buildings (eg, Uffizi) have elevators. You have to look around and ask. "Lift" seems universally understood. When there's no elevator, prepare for lots of stairs. Milan might also be a good bet. Flat, with lots of transit. And among hill towns, you might want to look into Orvieto. Once you get to the old town (via funicular and then bus), I seem to remember it being pretty flat; others who have spent more time there may have a better take on that. Make sure you know where the taxi stands are, and if possible, get a hotel very near one. (Or stay in one of the better hotels, with a doorman.) It's hard to flag down taxis on the street. The challenges with metro stations include no or broken elevators/escalators, and some long walks to transfer. Train stations often have a tiny elevator at the far end of a platform. Book long transfer times on trains, so you have time to get between platforms, and be at the door ready to get off the train when it stops (most stops are very short). And ask for lots of specifics from hotels. Often they'll say there's an elevator, but it doesn't go all the way to the room, or you have to climb a flight of stairs to get to lobby. If you're on a budget, convents are often a good solution, since there are older sisters who need an elevator; in places like Venice, with very few elevators, I've seen them in convents.

One other thought: Padua is relatively flat, less crowded than many cities, and has a convenient tram that takes you from one end of town to the other (ie, around from the basilica of St Anthony to the area around the Scrovegne chapel, w Giotto's amazing fresco work). You could also do a day trip from there to Venice, maybe just focusing on a ride down the Grand Canal and maybe one site of particular interest.

There is a ramp at Venezia Santa Lucia station. All the way around the left, as you come off the train, out the side entrance of the station is a path down to the Grand Canal, with no steps.

I posted to your question in another forum so I won't repeat it here, but the cities suggersted are good ones. I would add Bologna, Ravenna, and Ferrara. Orvieto is easi if you stick close to the Duomo area, and there are mini uses that will take you on a circuit of the town to the funivia. As you go further south, it gets a little more hilly. By the way, I was in Italy last summer and most elevators/escalators were working, and some major train stations have elevators that are more centrally located, although most are indeed at the end of the platform.

This is a duplicate question. Answers have been given in the General Europe area.

Or, try a bigger city and use the hop-on, hop-off buses for your transportation. The stops are located at the sights, and you don't have to go underground. It's an expensive way of getting around, but would tremendously help with your walking restrictions.
Try doing London and Paris.

Thank you all so much for your help. I will look into all of the cities you guys have mentioned.