1. McKeesport-Cedar Creek Gorge loop. 50 mi.
http://ridewithgps.com/routes/456821
Road ride on east side of the Mon, over the hill to Cedar Creek Park, and GAP trail back along the Yough. I've tried this - it's nice on a Sunday morning, when road traffic is low. The Mon valley roads (beyond Elizabeth, anyway) are greener and nicer than you'd think. It's not shown on the map, but a detour up into Cedar Creek Gorge to see the wildflowers is worth the time.
2. McKeesport-Fayette City-Perryopolis loop. 70 mi. A longer variation on #1.
http://ridewithgps.com/routes/830490
Roads along Mon to Fayette City, then over the hill to Perryopolis, then GAP trail back. When I did it, I crossed the Donora-Webster bridge, visited the Donora Smog Museum, and also the Big Barn Country Store in Perryopolis.
3. To Connellsville the hard way. 100+ miles if you do the whole thing (so I suggest shortening it).
http://ridewithgps.com/routes/1513557
Take roads (hilly) south along the east side of the Yough, then return via GAP. Route could be shortened by: starting in McKeesport or Boston, and crossing the Yough to the GAP by bridge at Sutersville, West Newton, Smithton, Layton, or Dawson. I haven't tried this yet, but would like to.