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.