Tomorrow is a rest day on the Tour de France ,if it was me that would mean a sleep in and a day lounging around the pool doing absolutely nothing. Then again I am no athlete and and any type of exercise on a day off isn't a day off in my book. That is what rest is surely all about trying to recoup your energy and rest your tired body. A professional cyclist is a different beast though and is not really a rest day as we know it.
They may have a short lie in, but most will spend two to three hours cycling. This is to prevent any lactic acid building up in the muscles making sure they are ready for the next day of the Tour. Normally the next stage is from the same town as the rest day so no travelling is required.We tend to forget they spend time checking in and out of accommodation every day as the race doesn't stand still.
Once they have finished the bike ride they will have a healthy lunch and spend the afternoon doing sponsors commitments and any press conferences they have to do. This sounds like a day full of things and not really a rest. For them though this is more about a break from riding for hours on end every day and the tour has two such days for them to relax.
When I say a few hours of riding it is not at a high temp but it is just about getting the legs moving. Every cyclist has a different type of ride and Alberto Contador used to basically do another race leg as his gentle rest day ride. His words were that his body is used to this and he hates to change routines. he will ride a few hundred kilometers for his rest day ride whilst others will do 30 or 40 Kilometers at a slow tempo.
These are super fit athletes even if some of them are juiced up and covering sizeable distances day in and day out isn't a problem.