The Dells are nice but busy as you know, Look at Mirror Lake state park or Devils lake and day trip to the Dells. The family did Custer state park last year and enjoyed it. If northern WI is your thing you cant beat Firefly and Crystal lake in the state forest up in the Minocqua area (was our goto spot for years)no electric but you can use a generator. Sleeping Bear in MI is neat for a few days visit and has several stops along the way to check out different sand dunes at various parks (camping in MI state parks stinks in my opinion, better off finding a private park).