I have a Triton, not a Nitro 929, but have similar storage bins. I keep my BPS rain gear and my kids life jackets in one of them. My GF keeps her fishing bag, my extra bag-o-baits and jumper cables in the other one.
I place a garbage bag around one of the containers for a trash bin. The other just depends on what I'm doing that day, extra towels, snacks/drinks, basically a catch-all.
I keep my boat cover and motor cover in there, since I store my boat in an uncovered space and when I uncover the boat, I have no place to keep the covers until I get back from the lake.