William O'Brien has two different campgrounds and we ended up staying at Savannah, and it was a really nice little campground. Most sites are relatively secluded, except mine which was kind of wide open and the location of a firewood drop box (74E). It was super large, though, so if there was a bigger party you could easily fit a few tents. The campground was really nice, but I'd probably pick a different site next time. 

The hiking at the park was fantastic. We did one longer hike that had some surprisingly good views. It was hillier than I thought it would be - not a tough hike, but hillier in that you could experience some nice views. The second day we did the river side hike, which was also very pretty but was also much more popular. I'd recommend an early morning visit to that one, especially during these Covid times.

Unrelated to the park, if you forgot any supplies, the general store in Marine on St. Croix was adorable and had really good sandwiches. Like another review said, either way I'd recommend driving in from that way regardless - the winding road and the tree canopy is really pretty.