Tons of options:
1) The Shaw Festival at Niagara-on-the-Lake. One of the premiere theatre festivals in Canada. (And it's not just for Shavian scholars.)
http://www.shawfest.com/
2) Winery tours. There are some very good wineries in the Niagara region. A cycle tour through the area can be great.
http://www.niagaraworldwinetours.com/
3) There is a great deal of War of 1812 History in the region. (Remember, that's the one where we burned down the White House...

)
Fort George and the Battlefield House Museum in Stoney creek are worthwhile.