It might be best to just formulate a few general ideas for the return trip and then play it by ear after seeing how things go on your vacation.
One possibility I personally would suggest considering is returning via I-70 and stopping in St Louis--specifically, I'd recommend the City Museum, under the assumption that probably a couple of your party of four are between the ages of, say, 6 and 18 years old. There are, of course, many other excellent things to see and do in St Louis, but the City Museum is a lot of fun and absolutely unique.