Even when you do just legs, you still work your central nervous system heavily.  That's why I do full body.  It allows the body to recover as one.  If you work out with sufficient intensity, I promise you won't be missing the gym that much.  Yoh will be so beat up that 2-3 days of recovery will be neccesary.
With regard to what I do on rest days, I do cardio.  But not running or anything to intense.  I just do lots of walking.  Very low intensity but helps with fat loss and recovery.
When a guy goes to the gym 5-6 times a week, all that signals to me is they either don't work out hard enough or they use PEDs.  A natural lifter to trains hard doesn't need more than 3 days per week at the gym.  With 2 days being sufficient if you want to maximize recovery.