Water is always the most important thing to have. If you have one a canopy will be great.. every time i go paintballing i'm always jealous of people who have those nice things for shade
tools in case anything ever goes wrong with your gun...this has happen me 2-3 times with my A-5 and the tools was very useful.
Chair is pretty nice too and maybe some flops to change out of your boots when your done but that is just me, i always wear flops
oh and if you are going and want to try out using air i'll bring my extra 72/3000 air tank for you to try