For me, a lot depends on the weather. When I take a rest day, I like to rest and relax. At this point in my life I am more discriminate about which sites I visit. I am not so inclined to visit some deserted crumbling monastery after seeing probably close to a hundred of them. There are a few favorites, but at this point I much prefer to visit an active monastery.
On rest days, I literally rest! I breathe in the beauty of a gorgeous setting, stroll a little, sleep late, eat well, talk to-other pilgrims and just relax. If the weather is good, I take a days rest in Acebo. It is not necessarily because I need a rest day then. It is because I love this place, and the scenery that surrounds it. But I only book a second day if the weather is good and there is an opening.
Another option is taking two short walking days instead of a rest day. So I split 20km into 2days, not necessarily together...and have extra energy for things I want to do. It helps break up the grind.
Albergue La Casa Del Peregrino in El Acebo San Miguel