Timer. Make it adjustable and try it.
We played a scenario game with one that went off in like 6 hours.
The trick is we had to find all the props that give us the CODE to disarm it. Then we have FIND the bomb, fight our way there, and keep our engineer alive long enough to disarm it.
He just had double throw switches that were color coded. About 5 of them all wired so if you did not flip the right switch in the right order it would bypass the timer and set it off. It was a smoke grenade with a Estes rocket motor igniter stuck in it. The props were CDs that were color coded and with the number of the order it should be switched marked on it with a Magic Marker. If you had only 4 of the 5, you had a 50:50 chance of getting it right. If you had only three, you were Very likely to guess wrong.
I think that if you captured and tortured the General or XO, they would give you the code as well. Tough mission even with 6 hours. We scoured the woods looking for those 5 disks and at least one of them was DEEP into enemy territory. The enemy couldn't touch it or cover it but they could leave a squad of guys guarding it.
Carefully planned irresponsibility is the KEY to mental health.
If you haven't grown up by age 50........
You don't have to......