Like
chazpat says, not may trail bikes will have a bar bag. At least not on the type of trails they're typically ridden on. And you mention quick rides at "parks"... like bike parks? Again, not where bar bags are common, for good reasons.
Are you fixed on a bar bag to address your goal? If not, read on...
I use a
Ortlieb Micro Two saddle bag. It works perfectly with dropper posts since it toes not attacth to the post in any way. And despite having four CO2 cartidges, CO2 inflator, Crank Brothers multi tool, two tire levers, and a rag to keep things from rattling, it fits completely under the saddle. With all this loaded, I only use about two thirds of the bags capacity. For longer rides, I do stuff a thin walled spare tube in it. The bag has a a quick detach, if needed for security. Ortlieb makes a larger version, the
Saddle-Bag Two. It's overkill in size, in my opinion. In addition to CO2, again for longer rides, I do carry a pump on the frame. A simple Spin Doctor RXE Mini Pump, which mounts under the bottle cage. To be honest, I forget its on the bike, so it's usually there for all rides.
Another consideration... do you use a water bottle? If not, a water bottle is great for storing stuff on a ride (obviously if you're not using it for water). I used to do this, but I do actually use my water bottle for water from time to time.
Anyway... some options.
