My sister and I will have 4 days in South Dakota. I've read other posts on this forum....but want to know (in addition to the Mt. Rushmore, Badlands, etc.) if there are any "must-see" or "must do" activities that are 'off the beaten path". We are on a tight budget--any suggestions on where to stay/where to eat? Is staying in Rapid CIty (where lodging is cheap) and doing day trips feasible? Thanks for any and all advise!