Growing up in the Chicago north side and suburbs, Wisconsin was the #1 weekend destination. My family had a large contingent in Portage so we were in the Dells often. I wouldn't worry about the cold in July, but mosquitos can be a problem. There are several big fireworks displays on the 4th so you could go a little early. I remember tours in Army Ducks (WWII landing craft) I bet they still have those, even after 60+ yrs. Real off road and off land trucks.
There were and likely are water ski acrobatic shows. Off road trails weren't a thing in the '50s but there are endless forest logging roads.