I have several RC aircraft that came with factory-installed drops. A B-17 & B-25 with remote bombay doors - even included little foam bombs and parachuted army men. I have a F4U Corsair with remote bomb drops for the included (& very detailed...too nice to drop!) bombs.
I've seen a Corsair with remote "rockets." The rockets were foam covers that slipped over bottle rockets removed from the sticks. It looked to be a real PITA to attach all the igniter leads!
I once watched an amazing B-52 with a remotely launchable rocket-powered X-15 slung beneath the wing! The X-15 used an Estes model rocket engine. It was very cool.
I think a remote water balloon drop would be kind of cool.