Shipshewana has a lot of activities, including a water park, Amish restaurants, and, of course, the flea market. I haven't been over there in about 10 years but was not impressed with the flea market when I went. Being familiar with flea markets in the south, this one is a lot different with new, and to me, overpriced items.
Throughout a good part of the summer, the 4-H county fairs are running all over the state. That's a good activity to do for a staycation.
State parks are wonderful! If you're into camping, it's quite inexpensive. Pokagon has 2 swimming areas, camp sites, the Potawatomi Inn, nature trials, horseback riding, and different craft of nature programs at different times. Bixler Lake in Kendallville is a city park with 2 swimming areas (with lifeguards on duty), camping, nature trails that are marked with education info about the different areas along the trail, fishing, and some activities as well through the parks and recreation department.
There are lots of museums scattered around the state as well. Auburn has the Auburn, Cord, Duesenberg museum. They also have a festival in the summer and the Kruse Auto Auctions (pricey collector cars) and car show. Rome City has the Gene Stratton-Porter State Historic Site. Ft. Wayne has several museums, The Children's Zoo, and more.
Of course, there are all the lakes up in the nothern part of the state with lots of camping, fishing, etc. Lots of small towns have festivals that are fun for the whole family.