I use a $5 zipper-equipped IKEA Fracta; it holds everything except the boat and paddle, dries pretty much instantly, and is colored brightly enough so that I might not forget it. IKEA also used to sell Rensare dry bags in a large size for $6. The small $3 ones are good for phone/wallet/car keys.
The best (worst!) was when a friend forgot to bring her paddle. We each took half of mine, which made for entertaining watching, I’m sure! Paddling our kayaks like canoes, we went half as far, but enjoyed time on the water on a stunningly beautiful afternoon.
On a camping trip where my girlfriend and I were going to spend a day paddling in Maine, we were a couple hours from home by the time I realized I forgot to pack my watershoes. Luckily I found an older pair in my vehicle, though finding them did solve the issue of the “mystery smell” I had in there. I thought something had just crawled into it and died while it was parked for a few months.
Since I switched from a pick up truck to a trailer I find it convenient to keep all my camping and a lot of my paddling stuff stored in the trailer. So it turns out I have a lot of extra stuff that I carry every time I head out. This spring I had a zipper blow out on my Timberline 4 tent. I’d repaired it a number of times and that zipper was nearing the end of its useful life. Fortunately I had a Timberline 2 stashed in the trailer and was able to swap the tent out before the torrential storm hit and I had to “hole up” for a longish storm.
I always carry an extra rain poncho in the “unhappy bag” (with compass, fire starting stuff, knife, TP, first aid kit, etc.) that goes with me every time I paddle. I’ve gotten caught out on the water in storms often enough to learn there’s invariably someone in the group who needs to borrow one. Good to have an extra and it doesn’t take a lot of space. A little extra food of some sort to share/trade never hurts either.