My go to park was st varian state park just north of Denver. I stopped trying to go there after Colorado changed the reservation policies, it became almost impossible to get a site. Its worth a try for you however great park.
https://cpw.state.co.us/placestogo/Parks/stvrain
Two years ago I stayed at cherry creek reservoir state park. This also is a great park south of denver.
https://cpw.state.co.us/placestogo/Parks/cherrycreek
A few years ago I had a hard time finding a spot in the foot hills and stayed at the KOA in central city. Its a great park but the elevation was to much for me to handle. I totally understand wanting to stay in the lower elevation.