I always carry: Vise grips, cresent wrench, 3/8 ratchet with sockets (including spark plug socket) and extension, hammer, channel lock pliers, needle nose pliers, screw drivers, crow bar, C-Clamp, continuity tester, file, tire gage, funnel, utility knife, electrical tape, duct tape (don't laugh it's a lifesaver in a pinch), bottle jack, various nuts, bolts, screws, washers, cotter pins, etc.
I also carry a boat winch that I can chain to the trailer in case the ZTR quits. That way I can get it back on the trailer by myself if it isn't running.
I'm sure there are other tools that could be helpful.