Yes, the wildlife loop in Custer State Park is a “must do.” We enjoyed Bear Country USA but it is a zoo. The first half is a drive through area where the bears are located and the second half is more of a conventional zoo that you walk through.
Elk Meadow RV park is a great choice for visiting Rocky Mountain NP because it is so close to the park’s entrance. Be sure to drive the Old Fall River Road if it is open, spectacular scenery. I believe it ends at the Alpine Visitors Center. You can ask there about locations of Big Horn Sheep etc. If that road is closed the Trail Ridge Road will take you to the visitors center and it too has spectacular scenery. We usually drive up on the Old Fall River Road and drive back on the Trail Ridge Road.
I took this photo of some Bull Elk on a snowpack at a pullout near the top of the Old Fall River Road. This was early July.