The Camp des Milles? It's very interesting and accessible by bus from somewhere near the center of town--or was in 2017. I think the bus may not run super frequently, so check the schedule carefully in both directions ahead of time.
The camp is a well-documented memorial/museum now, with virtually all the info translated into English. I didn't have quite enough time to absorb everything without rushing, so I'd recommend not making this the last stop of the day. On a revisit I'd allow at least 3 hours (I'm slow in museums).
I don't think there's any food in the immediate area--maybe a soda machine at the museum, so plan ahead.
The lower level of the museum was quite chilly even on a hot summer day, and you're not down there for just a few minutes. I'd recommend carrying a warm layer with you.