Bag for holding valuables on bike while parked?
Leaving valuables on the bike sounds like a very bad idea. Bike luggage just isn't going to be robust enough to resist theft if left unattended, and the whole bike itself may be vulnerable to theft.

What you want is a bag or caddy with a quick release so you can easily detach it and take it with you when you leave the bike. Many saddle bags have a quick release and come in a range of capacities.