Hi, We just came home from an InnerSea Discoveries Ultimate Adventure. This trip is two weeks long and combines the Eastern Coves and Western Coves. We have been to Alaska four times over the last several years. We are in our 60's, and yes, most of the people in the group were over 50 (but very active), but we also had some younger 20 and 30's on board. We have been on three Alaska Cruises, a 2,000 passenger ship, a 140 passenger ship and InnerSea's Wilderness Adventurer 60 passenger ship. By FAR InnerSea's is the way to go.
As to activities, they are all guided and planned and mainly depend on where you end up going. As example the Western portion the itinerary was changed to avoid predicted high seas in a more open area. Instead we went to two small coves where we hiked and kayaked. On each week they offered at least one 5 hour kayak trip where they packed your lunch for you.
There were two days each week with no planned off the ship activities, one day you whale watch and one day where you are at a glacier. They did however launch skiffs on both these days for up close views of wildlife and the glacier. You can also pay extra and camp one night on each week. You eat dinner on board and then they put you ashore with camping gear and a guide who cooks you breakfast.
We saw bears, whales, moose, eagles and sea lions up close and personal.
I would also suggest that you extend your trip with a train ride on the Alaska railroad and stop in Denali. If you can afford it, I would also add on a lodge at the end of the road in Denali. The lodges are all inclusive and have guided hiking options each day. We have stayed at Denali Backcountry Lodge and recommend it http://www.denalilodge.com/. The company also has a travel arm and they will book the train and any other activities you desire. http://www.alaskadenalitours.com/. They will customize a trip for you and then do all the booking. I used them to book a three week land tour a few years ago and they were great.