It really depends on what you are looking for. The Notre Dame campus is beautiful to walk around, even if you aren't a Notre Dame fan.
The East Race waterway is a small (just under 2000 feet I believe) white water facility located in downtown South Bend. Even if you don't enjoy that activity its fun to stroll along and watch.
Since Notre Dame is right there, there are MANY restaurants/bars, ranging from your traditional chains to independent small restaurants.
The College Football Hall of Fame is in South Bend, along with many museums, parks, a chocolate factory that gives tours, a zoo, and much shopping.
Depending on the exact date of your visit there may be a festival or event going on in one of the many parks in the South Bend/Mishawaka area. If you can provide your exact dates and more of an idea of what you are looking for in terms of activities, it may help.