If you want to see something extraordinary and have time to drive around then I would strongly suggest Carnac. It's a nice day trip and it is free unless you choose to go to the museum or take the little train or go on guided tours.
The standing stones (menhirs) number to about 3,000 but they may have originally have numbered to as many as 6,000. Many areas are fenced off to protect the stones and the native vegetation, but you can freely walk around on the paths. One area even has a little tower you can climb to get a better view. But be careful, this is not the USA where everything is clearly marked and the steps and paths are clean and safe. You do need to watch your footing and any children you may have with you. If you fall or hurt yourself...eh bien, tant pis pour vous (oh well, too bad for you)!
Most of the stones are between highways D781 and D768 intersecting at Plouharnel. There is a visitor center there. Go to monuments nationaux for the best information.
Alignment: rows of standing stones
Cairn: stone monument covering burial places
Dolmen: Breton word for stone table. A chamber sometimes with a burial passage, bounded by horizontal stones.
Menhir: Breton word for long stone or standing stone
Karnag: Breton for Carnac
You can click on the pictures to enlarge them.