Year Round
There are so many campgrounds. Those accessible by car are Colonial Creek (May - Oct), Goodell Creek (all year), Hozomeen (June - Aug), and Newhalen Creek (May - Sep)
There are also 15 additional campgrounds operated by the forest service, 17 campgrounds accessible by boat, and 4 campgrounds on Diable Lake
This park kind of reminded me of Great Smoky Mountains National Park if you simply replaced the black bears of the Smokys with the Sasquatch of the North Cascades. Really, there are bears here as well, but the North Cascades are notorious for their Sasquatch and they go as far as painting Sasquatch footprints on the highway to make their point.
I barely touched the surface of what can all be done in this park. There is so much water for paddling. All types of water too. I saw rafting expeditions, a convoy of stillwater kayakers, and even the casual innertuber on one of the creeks flowing from out of the mountains. Like most parks in the mountains, there is an abundance of waterfalls and hiking trails leading to them. Perhaps the best feature of this park is that there is a multitude of activities for the summer camper with relatively few summer campers that you have to deal with, but you'll still have to watch out for the Sasquatch.s