Hmm I might be a bit in the grey as I've never attempted sandbag training myself but I think it's really anything you feel like doing with your sandbag. If it's reasonably heavy (I'm thinking at least 70-80 pounds), doing sandbag overhead presses (without using any straps that might be on it, but only "pinching" the sandbag) could prove quite challenging. Actually it should be hard to even get it up to chest level as it'll slip around a lot. I've also heard of people attaching a heavy bag to a rope and then dragging the sandbag as fast as they could across a field (or you could also carry it, that would be much better for your grip). And as I write this I'm thinking you can do some sort of circuit training with that and some other things to focus on a good grip strenght... Just a suggestion though.
Maybe it could go like this:
repeat 3 times, rest 3 minutes between circuits and no more than 20-30 seconds between exercises, to be done in an open field with something to hang from nearby
2 minutes skipping (just for the sake of putting some cardio in there)
Sandbag carry 50 yards
Complete five full sandbag overhead presses (no cheating!)
Towel chin-ups, to near-failure (stop 1 rep short of failure)
One armed hanging, to near-failure
Farmer's walk, to complete failure (if you own dumbbels)
That should really pound the crap out of your forearms.
peace.