The dunes is a great option for guests of any age. There are closer beaches to lake Michigan like West Beach, but you can't beat the trails at the Indiana Dunes National Park. However, West Beach is still a family favorite. Here is a link to more information about the dunes: https://www.southshorecva.com/things-to-do/indiana-dunes/
Downtown Valpo is a vibrant area with lots of shopping, dining, and drinking options. My family and I like going to Tony's for pizza, but there is something for everyone in Valpo. (Fun fact: My dad Brett grew up in Valpo and now teaches high school biology and chemistry at Valparaiso High School).
Looking for something to do between the wedding and the reception besides getting dressed up? You could swim at the hotel pool, grab drinks at El Salto, or eat lunch and shop at Bass Pro Shop.