Juneau is actually the capital of Alaska, but it only has a population of around 31,000 people. There are no roads that connect Juneau to the rest of Alaska, so if you're are going there it is either by plane or by boat. It isa popular cruise port.
Mendenhall Glacier
The most popular attraction for visitors to Juneau is Mendenhall Glacier. Cruise ships will offer a shore excursion to visit the glacier, but I would recommend using the local tram. You can catch a ride near the cruise port and for around $5 get a round-trip ticket. When we were there the shuttle ran every 30 minutes, so you could easy catch a ride over to the Glacier, then explore at your own pace and catch a ride back when you are ready.
Along the drive to the Glacier you'll probably see a lot of bald eagles and possibly other wildlife. At the Glacier itself you'll be able to walk around on numerous trails.
Other ways to see Mendenhall Glacier include float planes and helicopter tours.
Mount Roberts Tramway
Another popular attraction in Juneau is the Mount Roberts Tramway. The tram takes about 6 minutes and goes from the cruise ship docks up to Mount Roberts. At the top there is a small village, with a restaurant, shops, theater, nature center, and hiking trails.