I think the choices of Quebec City, Montreal, Toronto and Ottawa are probably your best choices.
This is what I think would be my order of options:
1) QC would be great this year because of its 400th birthday. Here is a link to a website noting the festivities http://monquebec2008.sympatico.msn.c...calendar&id=11

2) Montreal and Toronto are both good urban choices, though they are different in many ways. They both have good selection of tourist sites. In the summer, they have a litany of festivals and events to keep a visitors occupied. If you are looking for friendlier, I would err on Toronto's side. Both cities have good museums, though the AGO is closed while the Gehry Pavilion is being built.
Montreal has a more Euro-feel to it, whereas Toronto is a more "modern"
city. I give Toronto a slight edge as it has more events to attend (such as Shakespeare plays - Dream in High Park http://www.canstage.com/2007-2008/content.php?ID=64) and places to see outside the city- Niagara Falls; Niagara-on-the-Lake; wine valley). In addition, it has communities within the city representing various regions of the world (Danforth - Greek; several Chinatowns; Little Italy/Portugal).

3) Ottawa is scenic, but 3 days is a long time there!