Some rest areas are busy, some are not. Juniper Rest Area along I-84 in southern Idaho is pretty busy.
Connecting Salt Lake City, UT with Boise, ID, the rest area is largely frequented by trucks and RVs, not so much passenger vehicles. Try to get here before sundown if you want to find some open truck/RV spaces.
Juniper Rest Area doesn’t offer much aside from restrooms, trash cans, picnic tables and vending machines. There are some short trails that meander through Juniper trees. But other than that, it’s your basic rest area.
Idaho allows you to stay at a rest area for no longer than 8 hours along interstates, and up to 16 hours on other highways. They do not allow overnight sleeping or camping. However, when Sash and I stayed there, we stayed for 18 hours and no one bothered us.
For the most part, Juniper Rest Area is clean and well kept. When we stayed there overnight, the winds blew fairly strong.