Well in the winter, there is the Northern Lights, Chena Hot Springs is neat to sit in the hot mineral waters and soak and watch the night sky! Ice carving contests and other related winter stuff, even the Ski Slopes at Alyeska Ski Resort is neat to spend a few days around.
Snowmachines, Snow Cats and a host of other winter type of stuff.
Fall is nice because after the first frost or so, all the mosquitos are toast, the place is just neat. If you come up the last week of August or there abouts you can head into Denali National Park and see a lot of critters there.
Unless you like being treated like you are helpless, the Cruise ships are nice, but they rarely have locals on them to tell you about what you are seeing. The Alaska State Ferrry System is a fraction of the cost, goes the same places and you can take your car. Not to mention that the locals are on there and you can meet some really neat people and they can tell you all about the areas/towns as you come to them.
Fall colors are starting good about that time, if you bring a car up, you can drive to the North Slope and run the "ICE ROADS" like the Truckers! Had some friends up this fall and I took them up to the Yukon River Bridge... All the way up there he kept pointing out this turn and that hill from all the episodes he'd seen on the series over the last few years, I just laughed.
Anytime that you can come here is going to be fun, maybe not spring breakup, but even that has good places to go see.
