If I were you; I was to get the most of the benefit of this layover (Who knows if I will be there again?!) I was to begin my day with a Doha day trip before I get back to the hotel for some kind of napping and having lunch before I continue with sand dune safari (that should include dinner). Later; I get back to the hotel, shower, refresh, sleep for 2-3 hours, and finally head back to the airport. That is the perfect life.
Just make sure to book an airport-based hotel (like Oryx Hotel).