It sort of depends on what you are interested in. Juneau has some cool side trips from the cruises, dog mushing on the ice fields, trips to the Tracy Arm, or taking the tram up Mt. Roberts. If you like to hike, the tram and then a hike further up Mt. Roberts or back down is nice. From the tram stop you can hike up above the tree line, and have some wonderful views, and then hike back down to the base and see the full range of forest. I think the hike back down is 1-2 miles. A trip to Mendenhall Glacier is nice, and a fairly standard cruise line stop. It has nice hiking around it, and if you can, go around the Mendenhall loop road to Auk lake and visit Chapel by the Lake. The interpretive center at Mendenhall is good, and I love the design, it looks like it grew out of the rock.
For a really easy hike, to an old gold mine, I like the Perserverance Trail and Silver Bow Basin. The trail starts at the end of a narrow winding road at the top of Juneau (it makes sense, really) and winds along Gold Creek to mine ruins in Silver Bow Basin. Silver Bow has lots of hanging valleys around it and is very pretty. The trail is level (an old road) and the hike is out and back, about 3 miles in or six round trip. This is one of my favorite Juneau hikes, I just hate driving to the trail head now that they have a salmon bake near the trail head. The tour buses on the narrow road make the drive a little scary.
If you are traveling with your kids, a couple of fun activities for them might be to take the city bus out to Twin Lakes and fish. One of the lake has a fishing docks and they stock the lake with baby king and trout. It is a great kid fishing spot. The lakes also have a stream on one side with a big pink salmon run. If you are there at the right time, the humpies are bright red and you can almost walk across the stream on them. Right across the highway from the lakes, is the fish hatchery. I like visiting the hatchery, and they also have a large aquarium stocked with local sea life. From the hatchery you can usually see sea lions feeding on the returning fish. If you want to snag salmon, the beach and fishing dock near the hatchery are great places.
The other cool kid thing to do, is to take the kids to North Douglas and let them play in the tide pools. The name of the park escapes me now, but I can look it up. There is a short trail to the beach, and at low tide you can find anemones, urchins, hermit crabs, other crabs, and the occasional baby flounder in the pools. There is no bus service so you would have to take a taxi, or rent a car. It could also be biked to, but it is about 25 miles round trip, maybe longer.
Darn it, now I really wish I was going home this summer.
Chapel by the Lake, on a good day Mendenhall is reflected in the lake:
http://www.pbase.com/ageojo/image/31650015
http://cruises.about.com/od/alaskacruises/ig/Juneau-Alaska/Auke-Lake-in-Juneau-Alaska.htm
Mendenhall Glacier:
http://www.fs.fed.us/r10/tongass/districts/mendenhall/index.shtml