I worked just outside the park for 3 summers in the late 90s, and the experience was unforgettable. Wildlife, great people from all over, and of course a vast and awe inspiring landscape. I think the best time to go is late August/early September. Northern lights are visible at night (May-July it doesn’t really get dark), the fall foliage colors are comparable to Maine or Northern Minnesota, and by that time the mosquitoes are gone. It is cold by then though, so pack your long underwear! Of course it’s neat to go for the summer solstice, but the black flies and/or mosquitoes will drive you insane even if you use industrial insect repellent. Definitely a place to go for an extended trip, since the weather can be foggy/rainy/crappy for many days at a time & some folks go for a week and don’t see anything! If you’re camping (the best way to truly see Alaska!) beware of bears and other critters and never keep food in your tent!!

I’ll ditto what others have said – find the Denali Foundation! They are wonderful people who know the area well and care about the future of the park.