Just a head's up here for anyone either with already formulated plans to visit some of the parks or are thinking about it:
Specifically regarding the Western/Southwestern parks, visitation numbers are already off the charts. Those numbers are affecting everything from the wait times just to get through entry gates, to skyrocketing car rental prices, to campgrounds/lodges already completely booked up through the summer (not that cancellation don't happen) to parking availability at trailheads and overlooks.
Additionally, some of the parks have either already instituted mandatory, timed-entry reservations for access (Zion shuttles) or will have them in place for high season (Rocky Mountain N. P., Glacier N.P. and for Cadillac Mt. at Acadia). Some previous first-come-first served campgrounds are going reservation-only (Yellowstone). Add summer road construction hold-ups here and there, unexpected cancellation of all Western Cabin reservations at Bryce through June and various other snags.
The short of it is to make good use of the NPS websites for the parks/monuments you intend to visit as some of the changes are VERY recent and there may be more to come. I'm seeing some disappointed folks over on the TA forums who didn't get entry reservations, land campground spots, etc. and sure don't want to see that happen to any of you! :O)
I've only glanced at a few of the websites but here are the URLs for some of the biggies (I'm always amazed at the amount of TA posters who don't appear to have used these at all for their trips):