The trip would start at the Route 117 Canoe launch in Lincoln and go down river and finish probably at the Lowell Boat Ramp in Concord.
This part is about 4.5 miles long. You could include a side trip to the Old North Bridge and/or the Assabet River. These are at the end of the Lowell Boat ramp so you can budget your time to finish on schedule (if you have one).
You can also finish at the Old North Bridge but you would have to haul the canoes a good distance to the trailer (parked near The Robbins House).
Here is a link to a Google Map:
https://www.google.com/maps/d/u/0/edit?hl=en&hl=en&mid=16a4Pp8NKEGioMpGhGL7GSx4GHpC6V6fp&ll=42.466166554013654%2C-71.35646089128451&z=18
The Sudbury River is in green, Concord in yellow, and suggested portion of Assabet in blue.
To be more efficient, it might be good that as soon as the canoe trailer arrives at the landing that you offload the necessary craft and drive it to the Lowell Road Boat Ramp (see directions in Google Map, 4 miles 10 minutes). Another car should follow to bring the trailer driver back. This will speed things up.
Other sights include an island with a geocache, egg rock, North Bridge as shown on the Google map.