a bottle holder, which i usually put a gatorade in. they make one shaped like a bike water bottle the dollar place sells for a buck each. i get a clamp on one for my handlebar, or make something for the backrest i usually have on most of my bikes.

a bike lock that has the clip like thing that lets you keep it with the bike

lately a motorcycle type swing arm bag. not sure how much abuse they'd take on a motorcycle (vinyl), but amazon (and ebay) sell a pretty nice one that i get for bikes.

https://www.amazon.com/Leather-Motor...ingarm+bag+red

both places sell this same bag (and a slightly smaller version also). same company, they're out of black and brown at the time, but they sell those colors too.

a rear view mirror. i don't ride at night much so i get lights if i find something i like, but i always try to find a mirror.