Dave and Deb,
We've been to South Dakota twice and have visited the same sites that you have mentioned.
About DeSmet-if you go into the town you can either pay for a tour or meander through the surveyor's house where the Ingalls spent a winter, the Brewster schoolhouse, where Laura taught her first year as a teacher, which was moved to DeSmet, and a small gift shop. About ten minutes outside of town is the Ingalls family homestead. You have to pay $15 to enter the homestead area. You can also stay there in your rv for the night, no hook-ups, for $30 (I think). It did not take us tons on time to see what we wanted to, so we drove back to Route 90. We did not have to double back for this.