[ghaff]

One caveat. The America the Beautiful pass does not cover campgrounds that have a nightly fee.

In general, I agree about trying to go to parks off peak season (and getting away from the more popular trails). That said, your hiking and other options can be a lot more limited at many parks that get a lot of snow in the winter relative to other seasons even if you have winter gear. (I've been to Bryce in the winter and it was hard to follow a trail in the canyon.)