The problem with hidden gems is that, once published in a forum such as this one, in a book or anywhere on the Internet, they are no longer hidden. Therefore, they will attract people.
Wildlife is best viewed at early hours of the morning. That said, you'll find less crowds at the national parks if you are out and about right around the dawn hour.
But you still have to deal with closed roads. As has been said, Going to the Sun Road in Glacier will not open until late June. Many of the roads in Yellowstone are not slated to open until mid-to-late May, and some won't open until June. As also has been mentioned, campgrounds may not be open until the end of May.
Opening dates of facilities in Yellowstone can be found here.
Yellowstone Park Roads:
2017 Spring Road Opening Dates
April 21: Mammoth to Old Faithful; Madison to West Entrance; Norris to Canyon.
May 5: Canyon Junction to Lake; Lake to East Entrance (Sylvan Pass).
May 12: Lake to West Thumb, West Thumb to Old Faithful (Craig Pass), and South Entrance to West Thumb; Tower Junction to Tower Fall.
May 26: Tower Fall to Canyon Junction (Dunraven Pass)
May 26: Beartooth Highway
Glacier does not yet have an opening date for Going to the Sun Road, but last year it was at the end of June.
Donna