I see two options on a rest day, slow easy ride or the aerobic blast so popular in many sports. It's supposed to leave you invigorated, it's short and gets the blood pumping and the muscle working briefly, it's reasonably intense but not involving suffering or pushing yourself. It's supposed to be fun.
Most of my experience comes from contact sports where you're physically in bits for a day or two, but have also experienced pretty extreme oxidative/glycolytic stress as well as wear and tear from heavy impacts. It's nice to do something fun that moves you around a bit, the big muscles of the legs and back in particular kind of go "aaah thank you".
This is where good coaches stop being physiology robots and look at the human in total. Chasing your kids around the garden and tickling them would be an aerobic blast. Playing some casual badminton, doing keepy-upps with a football. Even a bit of golf. Swimming is great, I am pretty crap at it but it's just a great feeling when you're physically trashed. I wonder if the riders have a dip in the pool on stage races. I guess they have a professional full body massage, or several of them instead.