Another vote for the Takshanuk Mountain Trail 4x4, especially with a teenage boy. My then 14 year old loved it and still talks about how fun it was almost 5 years later.
Our favorite excursion was the Neets Bay Bear Watch by Float Plane. I know you said no helicopters, but this is by float plane so thought I'd mention it. We were probably 15 to 20 feet from the stream and watched bears fishing for salmon. There were tons of eagles as well. The hike to the viewing platforms is very short and on level ground.
The other excursion we did was the Alaska Whales and Science Adventure. Initially I was worried that my son would find it too focused on younger kids, but he loved it. He was the oldest non adult on there, but because of that he got to do the "dirty work", hauling in crab traps, hiking down to the river to take temperature samples. Honestly, he ate it up. Our guide was great and the boat only had about 15 or so people on it - so much better than those floating masses of humanity we passed. We also took a short hike to view the Mendenhall Glacier as part of the tour. We were on a small beach with a great view of the glacier with hardly anyone else around except our group.
Our son loved all three of these and I'd recommend any of them. All were booked through Disney. I hadn't really discovered online cruise forums at that point and we just stuck with Disney, so I don't know if you can book these independently or not.
