It's a little bit different for each genre of electric aircraft, I'd think. For foamies, you'd want packing tape and epoxy for quick field repairs, for example.

Generally speaking, you'd want most of what you'd have in a normal glow field box: Screwdrivers, pliers, extra props. A cheap multimeter can be handy to diagnose power problems at the field.

If lead-acid batteries weren't so heavy, I'd recommend a pair of 6V golf cart batteries for all-day charging.