Plan to be on deck for several hours when you are at Hubbard Glacier. And dress warmly. We were there in August 2009 and needed all of the warm clothing we brought, including hats and gloves. But you don't really want to do all your viewing from inside the ship. Being outside where you can feel the chill and hear the "white thunder", the creaking and cracking of the glacier, is part of the experience. And you will undoubtedly see and hear at least one "calving" while you are there.
I also highly recommend the whale watching expedition. We took ours in Juneau. The boat pilot and the "naturalist" were very good at what they did and we saw a lot of whales. We saw "bubble net feeding" and "pec slapping". We were much closer to some of the whales than is allowed by regulations, but it was okay since it was the whales that approached the boat and not the other way around.