The section of the Blackstone described here makes a very good short trip, because it has something for everyone. It has moderately fast water with lots of S-Turns, interesting historical sights to see, and as an extra bonus - it is a loop trip, so no shuttling of cars is needed.
Start the trip at the River Bend Far Visitor's Center in Uxbridge. From there a short paddle up the Canal takes you to a short portage over to the Blackstone River. The river itself is just exciting enough to hold your interest, but no white water skills are needed. (An exception would be in high water, when it can get pretty rough. Extremely low water late in the season also is an issue.)
Take out again about an hour downstream at the portage steps (there is a sign) River Right, and carry back over to the canal, from which it is an easy paddle back up to River Bend Farm.
Route 16 (SW route out from Boston) to Oak St., to River Bend Farm.
Put-in & Take Out at River Bend Farm.
National Park Service
National Park Service - Blackstone River Tour - River Bend Farm Tour #3: Link to PDF file