We've reserved a car for most of our trip but are deciding if when we want to pick it up. We will arrive in Marseille and stay one night, then have a driver and tour to Cassis the next day, with plans to be dropped off at our apartment in St. Remy at the end of the day. I'm thinking the next day we will just stay and explore St. Remy and not travel elsewhere, and so would get rental car on the 4th day. So, the question is, is walking around SR to see old town, cafes, etc. easy enough or so we need to have a car for that day to see SR? We are staying there 9 days but plan to drive all around Provence from that base most days. Hope this question makes sense!
There's more than enough to see in and around St. Remy by walking to keep you occupied for a whole day. We've spent a week in St. Remy on 2 occasions, and one of them w/o a car. While we did go on some day trips by bus, we also spent quite a bit of time walking in and around the town. And if there's a market that day, there's even more to do.
I don't believe that any major car rental agencies are present in St. Remy, so you'd have to take a bus to Arles or Avignon to get one. I would pick up the car upon departing Marseille and use it for Cassis. I'd expect a couple of extra rental days to be cheaper than a car and driver and the driving there won't be any different from the rest of your drive time.