I've sometimes done Yonge Street up as far as I can go (usually time constraints). This is good for an early morning ride on a Sunday. I used to ride Don Mills regularly for my commute to work but since work moved up to York U I haven't been on it in ages. I do use the trails on Mondays biking Westward as far as I can go and have also been doing a Lakeshore ride out to Clarkson and back (I live downtown so everything outside the core can be a nice ride
). One of my training rides I used to do (sometimes still do) is to take Danforth out to Kingston Road and take Kingston until it meets the 401. Then I'd turn around and head back.