I've been there a few times. I think it's a recreation area, not a national park, btw.
I worked in Rock Springs (North End of Flaming Gorge) and in the summer, it's light there til almost 10 pm. I headed up there for photos after getting off work.
Lots of wildlife, although it is fleeting. I mostly shot sunset photos off of Firehole Canyon Rd, although I drove in from the south a few times but only stopped to take a few shots along the way due to time constraints.
You'll do best with a 4x4 to get into the park as all the roads are dirt/gravel once you get past the boat launch areas. Lots of weather conditions, so each sunset is fairly unique. Very little cellular service once you get into the park, so make sure you take provisions (water, food, spare tire).
Could be good if you have a boat to explore. You may have to park it to climb the hills on the east side to get good sunset shots, which is why being in an off road vehicle gives you a good perspective.