Carnac

Carnac has three major alignments (vast fields of standing stones). A fence prevents you from walking among the stones because of damage done by thoughtless visitors. but there are guided tours (in various languages, including English) that permit visits inside the fencing. You can also walk, bike or take tourist busses along the road.
In my opinion it is the sheer size of the alignments that is impressive, rather than any one feature in particular. We've arrived by car each time we visited, but then got out and walked. You could do that regardless of how you arrived.

When we were there in May 2023 without a car, we stayed at Hotel Le Diana which is right opposite the beach for two nights. From there, we could easily walk into the village and the tourist information office for advice, to several cafes and shops, and took the Petite Train to get out to the alignments (twice!) which we caught by the beach just up the road from our hotel. We also took the public bus to the museum by the stones which you can also catch in front of the tourist information office. Hope this helps!

We hired a guide who had a car and crafted a visit to the neolithic sites we wanted to see -- he was very knowledgeable and a pleasure. https://janettravels.wordpress.com/2020/11/29/yes-ed-more-rocks-the-neolithic-alignments-at-carnac/. We had a hotel in Auray because Carnac hotels were booked up and he picked up up at the hotel and then took us to the station to catch our train back to Paris.

Our guide was SAmuel Clouet and he seems to still be active -- can't say enough good things about him.
https://www.tripadvisor.ca/Attraction_Review-g1940567-d7923648-Reviews-Visite_Bretagne-Janze_Ille_et_Vilaine_Brittany.html

I'm sure Carnac is excellent, but note that there are less impressive Megalithic stones (including single stones) all over this part France. I'm only referring to allocation of time.