Ontario place is a small amusement park for younger kids. They have a number of rides and even a water park. They usually have a number of people in costumes for you to take pictures with. There is also an Imax which you can watch certain films (but not movies!) for free. You can walk along the harbour and cross over to the Canadian National Exhibition centre during summer.
However, the park is getting a bit old and there is not a lot to offer for the older kids. Adults may find the place a bit dull.