The Indiana Dunes are always a blast! There are lots of things to do here such as relaxing on the beach, going for a swim, or going on a hike. Mason and I typically go to the state park portion of the dunes, but this section is closer to the venue/hotels. If you are into hiking, I recommend the Cowles Bog Trail (linked below). This is the trial my students and I hike every school year! It's about a 25 minute drive depending on where you stay.
This place is a little further from the venue. It is actually in our hometown Valparaiso! Mountain biking is one of Mason's adventurous hobbies. Trailyard is where he takes his bike to ride. The trails are really nice, and the Trailyard Outpost has bike rentals if you want to give mountain biking a try! If mountain biking isn't your thing, some of their trails allow for hiking as well.