I took DS6 to a tube park yesterday and we both had a blast! He also loved the Ontario Science Centre. Last March Break we did an overnight trip to Niagara Falls and went to the Bird Avairary and he hasn't stopped talking about that place and can't wait to go back.
Mountsberg does a Sugar Shack - a nice trek through the woods, ride on the back of a tractor - explore how Maple Syrup is produced. They have pancakes and hot chocolate. An easy day trip - not expensive and will definitely wear them out!!
dopeyluvsgrumpy - what tube park did you go to? That sounds like fun!
I was going to suggest Niagara Falls - a lot of the attractions are still open. There are some great hotels with cheaper rates this time of year as well.
Kitchener has a fantastic little children's museum. It is not too big but full of interactive elements. You do have to pay to park and I always packed a lunch as the little eating spot attached to it is kinda funky (and expensive) I think the wesite is www.thechildrensmuseum.ca or something like that. I think the actual formal name is waterloo regional children's museum. An excellent day trip and well worth the drive
There is a conservation park in Stouville where the kids can learn to make Maple Syrup, and then go to the childrens safety village which is quite interesting.