Mobile apps we found useful
visit a city
https://www.visitacity.com/
Enter the city and it will give you things to do for that city. There are various choices for itineraries or you can custom build your own. You can do that on the web site or on the mobile app. The best thing is the mobile app is location based. It knows where you are, plan the most efficient route for you and give you detail directions. Or you are at some location and wanted to know what attractions are near by, it will tell you based on your location and how to get there.
Rick Steves Audio Europe
https://www.ricksteves.com/watch-read-listen/audio/audio-europe
Rick Steves has audio walking tour. We did several of them and they are pretty fun. Remember to bring earbuds since it might be hard to hear the phone on the street. Also we keep our phones in our pocket and sometimes only use one earbud. We usually find a nice spot to sit/stand listen to what he has to say, then move on.
We used Google map all the time. You can download local maps so you can use them any time.
Google translate can be very useful as well. You can download local languages so you can use them any time. The camera works great for restaurant menus, supermarket items, signs, etc.
Train pass
We spent almost a month in France and Spain this April and did not buy a rail pass. As others pointed out, a rail pass is not too useful in those countries. For example, the Spain train Renfe requires reservation. The pass doesn't save you any time or money. You have to pay a reservation fee and have less seat choices as a pass holder.
We spent almost a month in Switzerland, Italy, Austria and Germany last September and we bought a pass. We never calculated if we saved any money which we might but we think the convenience is worth it. For trains that don't require reservations, you can get on/off the train freely. We used Eurail.com to buy our pass. Their mobile app Rail Planner worked well for us. The only train we needed to reserve was the high speed train from Florence to Rome. So other than buying the pass, we only had to buy one other ticket. But this all depends on your route. You can check which trains require reservation, which trains are reservation optional and which ones cannot be reserved. If trains are reservation optional, you can see how popular they are.
There are several train routes in Europe that are known to be very scenic. We fitted part of one in our Switzerland travel. If you like relaxing in the train looking out to beautiful scenery, look into it.
From train station, we have walked, took metro/bus, took taxi or Uber. Every city is different.
Lodging
We have stayed in Airbnb many times in many countries. The main advantages of Airbnb is more space if you get an entire apartment since there is usually a living area/dining area, sometimes have laundry facilities. Airbnb check in/out can be more cumbersome, cancellation policy might be more restrictive. When it is only DH and I, we would look into staying at hotels as well.
In case you want to consider hotels:
We stayed at a hotel right across from the Lucerne train station, hotel is very charming and location is good. Lucerne is a small town, you can almost walk everywhere.
We stayed at a hotel right across from the Munich train station, hotel is fine, location is fine. It is very convenient since the train station has a very large food court with all kind of cheap/good eat. Also the metro station is there as well, so you can take the metro easily but also within walking distance to some attractions.
We stayed at a hotel right next to the Zurich train station which is also the metro station and across from the airport. It is a huge station with restaurants, supermarkets, shops, etc. It is farther from the city center so you need to take metro wherever you want to go. We stayed there because we were flying out of Zurich otherwise we probably would stay else where.