Many of the tours do not have a lot of walking. You may take a tour bus or some other form of transportation and than walk around for a short time. Obviously you may want to stay away from hiking tours.
Skagway has totem park tours, some walking but it is on good walking paths. There are also horse drawn buggy rides as well as amphibious duck tours. Fishing and flight tours are big here but can get expensive.
Juneau you can take tours to Mendenhall Glacier. You can also pay $12 and ride the trolley to most of the town of Juneau and get on and off as you please. Juneau is great for helicopter and whale watch tours, again can get expensive.
Skagway there are tours into the Yukon. There is also the White Pass Railway. Helicopter tours are big here. You can walk the whole town in about 15 minutes if you don't want to stop in the shops.
Check out the Princess web site for tours they offer. They will give the information on if it is a tour with a lot of walking. You don't have to take their tour you can find similar tours by the locals if you wish but they will give you an idea of what is available.
