A good set of gloves (such as mechanix) is a nice purchase. And if your an active sort of gent then Knee pads can't hurt, even if you only wear one (on your dominant knee).
Its been said before, but water is important. Often your location on field will be removed from a water supply and keeping hydrated is always a good idea. Even if its just a simple bottle (although hydration packs are nice as they don't slosh and are easier to lug around) usually carry with you 2 liters per 8 hours if possible. And I know this sounds insane but carry something salty, such as peanuts or a granola bar with around 200mg of Sodium. Or substitute the water for a sports drink. Try to take it easy with water, over hydration is just as bad for you as under hydration drink when your thirsty.
Some other things that are nice, Environmentally friendly bug repellent
as it does not contain anything that can be detected easily and it won't eat through your plastic on your guns like bug juice will, also sun screen is nice.
Most everything else you can best determine by just trial and error. If you find that you could really use something that does something then you get that, if you find a piece of kit is useless then dump it. I do suggest though that you consider modular gear, as it will last you longer and will allow you to modify it over time to suit your needs.
Good luck and have fun.