The waterfront trail, from what I remember, is not very well connected through Scarborough - partially due to geography (bluffs) and partially because land access was not negotiated. Here are some maps of the "trail" in that area; as you can see, most of the "trail" is wayfinding along side streets, which I wasn't interested in when I was looking for cycling trails, but it could be good for longboarding... I haven't explored that end of the city for a long time. Maybe I'll head out there when the weather and my days off line up.
http://www.waterfronttrail.org/maps/wt-2_15.pdfhttp://www.waterfronttrail.org/maps/wt-2_14.pdfThere aren't really any paths from Scarborough to downtown, just little trails here and there that don't connect in any meaningful way. See this super-complicated map (ignore all the dotted lines; they're just "proposed" paths and bike lanes):
http://www.toronto.ca/cycling/network/pdf/2012-02-06_2012-02-06_trail_upgrades.pdfThe trail that you see from the DVP, however, does go all the way downtown and connects with the waterfront trail at Lakeshore and Cherry St. It's one of the few functionally connected recreational paths in this city, a legacy of hurricane Hazel. Some of the asphalt is pretty gnarly (especially the south end beside the river... lots of tree roots have busted up the trail), but it could still be pretty fun to explore.
http://ontariobikepaths.com/DonRiver.htm