As a resident and frequent traveler of the area, I could make a lot of recommendations, but this is a big place and there is a lot to see. It would help if you gave us some idea of what you would like. Is it scenery, hiking, museums, historic sites, tourist areas? There are plenty of all.
House on the Rock, is more hype than experience when we were last there, but Circus Museum in Barabo is worth the stop. Which lake is just another issue.
Matt