You will be fine with no reservations if you are on a bike. Some parks
especially Yellowstone will be booked but just drive around, find peeps on bikes and pop in, no worries. This will NOT work if you are in a car or rolling railroad car. The $80.00 National Parks is the best pass if you are going to many National Parks, good for many other places also. If you are on a bike, take the dirt roads, (legal),that Motorhomes, trailers and such are not allowed as you will be by yourself in beautiful areas. Stay for 2 days at least and take some of the dirt,one way so you might have to backtrack a little. Hayden Valley is the best for seeing wolves and grizzlies but there is wildlife all over, don't pet the bison!!! Don't miss the falls!!
Rode a 1967 yam 350 there on my 4 corners trip when I was 17, trail 90 when I was 19 from SoCal,1080 miles and was there 2 years ago on a tw200 from Boise Id and back, 10 days 1750 miles. Few times in between. Still one of my favorite places on the planet that I have been. Will be there this summer also.
PS. Bring your skeeter repellant!!!
Have a wonderful time.
worlds worst mechanic