What to do with ever increasing pile of empty CO2 cartridges.

Build a "potato" gun for them. 
Use standard compressed air ~100 psi.. and you can kill a deer with it.. accuracy may be an issue.. 

targets for plinking

Insert a 5" piece of hanger in neck. Spray bright orange, hang out at different distances and have at it. 60 yds with Springer. Hmmmmmm

I save all of the spent cartridges. Might sell'em for scrap? Or make slip bobbers out of them? The kind we'd tie on some 6-8 feet in front of a Shad Dart jig we used to catch big white bass on. Not sure exactly yet? 

Wind chimes

We would drill out the punctured end,. Then fill them with cut match heads. Little wick after stuffing them solid and you have a rocket. Use a pipe and light it off in a safe direction. 

Swinging targets at 25 yards. I also hang them in a tree at 34 yards on a string.

We hang and shoot them at 55 yards.

Filling empty co2 cartridges with match heads is risky business. A kid from my high school days did this and lost three fingers as he was stuffing the match heads into the cartridge. My dad always warned me to use only black powder and GOOD fuse, like dynamite fuse with a known burn rate. Back in the GOOD old days, blasting grade powder was 50 cents for a quart mason jar and waterproof fuse was a nickel per foot. That was big money considering I only made 50 cents per week allowance.