Travelling to Montreal

I'm going to Montreal at the end of April and will have my two young boys (5 and 7) with me. I have a couple of days to kill and would appreciate any suggestions for kid friendly activities. Last time we were there for just one day, we went to the Biodome - they really enjoyed that, so we will probably go back. We are also going to Ottawa, but I have been there several times and have a pretty good idea of what they will enjoy seeing there.