I just bring diapers, adult sized wipes, some sort of trash bags (if I'm wearing really thin ones a dog poop bag dispenser works and I have some rolls of slightly larger but still fairly small black trash bags if I'm using thicker diapers), and hand sanitizer.
I don't like baby powder and I don't think there's anything else I really need for diaper purposes, although I usually end up bringing water and a snack if I'm going to be out for a while.
Actually it just occurred to me that since I've been using a gold bond friction defense stick thingy (it comes in a dispenser like antiperspirant) where the leg gathers go around the inside of my thighs when I put on diapers in the morning maybe I should take that with me so I can reapply it if I'm out for a while, since I think it helps with thigh chafing a lot.