I often stop to take photographs but I really need my keys. Used to use a small top to bag but I’ve since moved instead to a small bar-bag. I like the bar-bag because can easily reach and see to retrieve things from it without requiring to dismount.
It also has a click fix mounting and a small handle on the top so I can take it off and carry into the pub/services/post office/etcetera with my keys wallet phone rain jacket all in one place.
The phone hangs in there suspended in a rear-pocket along with a small card wallet.
The keys are attached to the small carabiner which is sewn onto the inside of the bag. Rain jacket plus skullcap with puncture kit and multitool all live in the main compartment.
I like it a lot.