Summers in Denali are pretty awesome. If you get there early you may get to watch the ice break up. Kinda surreal to go rafting with icebergs floating downstream, sometimes they get in the way, but mostly they are fun. Lots or great weekend trips if you bring a car up - and the drive up and back is amazing, except for part of the Yukon which is fairly flat. Mid summer you usually get some really big flows - 30,000 + CFS, it is a lot of fun to be on water that big and fast think of 20' + waves. Late season you get to see the aurora Borealis - something everyone should experience.
I guided for Denali Raft Adventures in '02 and '03. If you are going up there PM me and I will give you all the beta that I have.