(Yeah you know it rhymes and it's catchy)
When shooting bb's and pellets has lost it's thrill try some of this ammunition.
A Shotgun Effect: load your gun and pump it or whatever and pour some uncooked rice down the barrel about 25 grains is more than enough. Pump your gun about 20 times and watch the blast. This rips a cardboard box to hell. If you don't have rice try cylinder shape sprinkles. Or tiny plastic beads. They all have about the same effect.
Another fun thing to do is load strike anywhere match stick down the barrel. Pump the gun and shoot at something hard. The match will explode with a nice crack sound to it.