At least one spare battery. For me the sweet spot is three, but could manage with two as the flight time has increased a bit over earlier models.
You can get chargers work on 12V too.
I would get a short conventional USB to whatever-your-phone-uses cable rather than mess about with the short flat ones they supply.

Think about how you will launch, with practice you can hold it to start, but there is some risk, hand catching is rather easier.
A car floor mat is handy to put out - launching from car roof etc is risky due to magnetic interference.
You can buy pads that fold out but most farms will find something they can use already.
Launching from a dusty or wet surface can mess up the camera and won't be good for motors, grass is not ideal unless very short and dry..