I am renting a canal boat in Messac in May of 2023. I will have one full day to travel from Paris to Messac, which is a 3 hour drive. I would like to see prehistoric sites on the way that is not 3 hours in the opposite direction, as there are none along the canal boat route. Does anyone here know of any menhirs and/or passage tombs sort of on the way from Paris to Messac?

Google Menhirs near Messac and Menhirs near Rennes. Google will map a few.

There is a good alignment at St Just, not so far from your starting point. St Just, Ille-et-Vilaine, Brittany, France, Alignment - M E g A L i T h i A

We have visited it twice now.

Also worth a visit is La Roche Aux Fees in Esse, the largest Dolmen in France. https://www.ille-et-vilaine-tourism....oche-aux-fees/

Enjoy your boat trip, the canal is lovely!