It depends on what I'm doing. If I'm just putting around on a Sunday, I tend to take a "less is more" approach. If it's a contest day, or I'm working in my planes for a contest, I'm more likly to bring the shop with me. I have a second "repair box" that I take that has all the extra stuff in it.

You can reduce the amount of tools and parts you bring to the field by doing a solid pre-flight the night before heading to the field, and a careful post-flight when putting the plane back in the car. Check all those screws and joints and bits for looseness, it's amazing how many of them you can find before stuff falls off in the air.

A water bottle is important on hot days to avoid dehydration and in case someone gets dirt or fuel in the eyes or to clean out a cut.

I have a fast field charger I really like. Allows me to quickly put more juice back in the RX battery at the field. A bit pricy for someone new, but worth it. I've charnged more than one newbie's gear when he showed up with a badly charged pack. (I have an FMA Supernova 250, and there are a lot of newer options that are as good or better).

I try to have an adjustable wrench in the box all the time.

Paper towls. For clean up afterwards, and for all kinds of clean up during.

q-tips. Great for cleaning out an engine after planting it. Also good for getting all kinds of junk out of those hard to reach crevices. (I suppose most folks don't plant an engine then clean it out and go back to flying the same day, but it's pretty common in combat)

A plastic shopping bag for garbage, used rubber bands, oily paper towls and other bits of trash that seem to occur. I do think a full size garbage bag is a bit morbid .

It's been said, but it's worth saying again, first aid kit.