How about adding a "defusable bomb" as reported by Mako Events as a mission objective to your airsoft game. Either you plant this bomb or defuse it and if you have to defuse it, you only have 10 seconds to defuse it. The wires can be randomly set so that there's no pattern how the wires are arranged for you to defuse it quickly. Either cut the wires or you can unscrew the wires.

Lots of details at the Defusable Clock website so you can understand how it works. But this kind of device, even it's a dummy one can create a lot of havoc and panic as it is if shown in public. Always store this away during transport, and we'd rather suggest that you don't carry this to an airsoft game using public transport (especially on a plane). This will surely land you in jail and much worse, may just carry an even stiffer penalty  for creating a public disturbance.