For us last summer, it was easily the Kenai Fjords National Park boat tour from Seward. It was my third trip there in 20 years and Major Marine had by far the best tour skipper I have had on those trips. And we had no reservations until the day before we took the boat trip. We just pulled into our campground, unhooked the trailer, drove into Seward, and walked up to the ticket office with our coupon book. Next morning we were on the water.
I wouldn't try to do that over July 4 weekend in Seward however!