Prior to the tour, I like to calculate that I'm going to take a rest day every 4 days ... 4 days of cycling, 1 rest day. It may not end up being exactly like that of course, but that allows for some flexibility.
Once I'm on the road, I usually try to take my rest days somewhere with tourist attractions. When I toured Australia in 2004, one rest day was spent on a cruise in Tasmania and another was on a cruise in Queensland. On a tour in Europe, Rowan and I took in the museum at Ieper on a rest day. One of the best parts of tours is being able to stop and taking in the surroundings.