I prefer HIIT training over traditional conditioning. It simulates a wrestling match more closely than long-distance running because you have sprints and short rest periods. When I do it, I do intervals of either 2 minute sprints and 1 minute rest or 1 minute with 30 seconds rest. You can also do this with a jump rope if you have one.
As for body weight training, I would only do this if you have no access to a gym. Regular weightlifting, especially olympic movements such as power cleans, are ideal. At my school, we usually do 5 X 5 of the four main lifts - bench, squat, deadlift, and clean. We do the minor lifts if we have time.
A good way to train your grip is to buy one of those grippers and use that. Some alternatives are tennis ball squeezes and plate pinches. My favorite for grip is towel hangs (if you have access to a pullup bar). Deadlifts also indirectly work your forearms.