I can tell you from my own experience River, props, props and more props!!
I was flying during vacation in Arkansas last year (my p1 thankfully, not the p3) and got a little cocky while flying up a deserted road and crashed hard breaking two props. At the time I only had a single set and spent the next 4 hours trying to find a hobby shop anywhere near our remote vacation spot. I now carry 4 spare sets with my P3 kit.
Beyond that, I'd say spare batteries for sure. Maybe some spare SD cards if you fill those up quickly. I bought a power inverter so I could charge my batts on the go. That's all I got for now!