I've recently gotten into climbing as kind of an off-season activity. After a couple trips to the local climbing gym, I noticed that distance was a little more effortless. One of the by-products of climbing is a stronger grip. I've found that I'm getting a stronger grip due to climbing and it's leading to unprecedented distance gains. That's my unscientific conclusion, anyway.
One day I spent an hour climbing and then went straight to a field. This time of year, daylight is scarce. My forearms were completely toasted and I tried to throw a few shots anyway. Not unexpectedly, I didn't throw very good shots and the results were rather comical. Nothing worked because that grip was not there when I needed it.
I suppose there is some truth to my earlier observation. A great off-season sport to try is climbing. You work on balance and stamina, and as a byproduct, you get stronger in ways that benefit your disc golf game.