While underground rails are usually the cheapest way to get around large cities and the preferred option for locals, you miss seeing a lot of the sights that way. In addition, the flights of stairs down to and back up from the tracks can be painful if you have difficulty walking. (Elevators aren't always available at each stop.)
There are tour buses that allow you to hop on and off as often as you want in many major cities around the world, and many have wheelchair lifts. For some, that is a good option.
Since we didn't have the energy to do much "hopping," and it was much easier to get in and out of taxis than climb up bus steps with tired legs and a cane, we opted to take taxis as we toured. As much of a splurge as we thought that was going to be, it ended up being far cheaper than the city touring buses! Your situation might be different depending on where you want to go and how many are in your party, but for us the entire cab fare for the day didn't add up to the price of one city tour bus ticket. Apart from a small base fee, you pay by the mile/km so you can get out where you want and find another one when you're ready to leave. Not having to hurry because others were waiting made it much more enjoyable. Obviously you see more in the front seat, but all three of us had window seats and thoroughly enjoyed it.
If you tell your driver you want to go the scenic route or past a certain site on your way somewhere, it might cost a little more to go slightly out of your way, but it is well worth it! It helps to have an idea where you are and how close you are to where you want to go. Drivers might not speak your language, so try to have the names written down in the language of the country you are visiting. Also, most won't tell you much about the sights you are seeing, so do a little research yourself ahead of time.
Tour buses are very nice if you will be traveling long distances or want to visit several tourist sites that are located in different cities. It is good to have a chance to rest while they worry about traffic and directions.
Tour buses will take you to the village below Neuschwanstein Castle. It is a beautiful castle that is on most Bucket Lists. Unfortunately, it isn't until you've gone up a long hill to the ticket window that you will see signs saying that the castle is filled with many, many steps and is unsuitable for people who have difficulty walking or climbing stairs. You can, however, enjoy the beautiful view of the castle from the village below. This picture was taken from there after we enjoyed shopping and dinner at one of the village restaurants.