Any suggestions for good family outing?

My sister has been spending the year abroad, and she's coming for a visit on the long weekend (april 9,10,11). We want to do something fun in Toronto as a family, with my parents and other sister. Since we are getting older now (ages 16-21), we haven't done anything as a family in a while.

Could anyone help me with a cool idea? It doesn't have to be the whole weekend. One day would be fine too.

Thanks!