Check the woods behind Woodside Public Library on Rebecca St. (in Oakville) I haven't been there since I was a kid and BMX-ing but there were some pretty good hits...
Not sure if anyone trolls this forum anymore, but if you're looking for a place that does good mechanical work / post season part changes, check out Rock and Road Cycle. They're a new mtb shop in the area and a bunch of great guys, corner of Dundas and Guelph line, in the Macs plaza. If you go tell em Kevin sent you, they'll help you out.