When I brainstorm things I start with drawing out a simplified wiring diagram. Most of the things we want a pack or other prop to do - such as venting- are based on circuits opening and closing. The opening/closing of circuits is managed with a switch or relay, there are different types of switches (NO, NC, momentary, toggle, etc) that will provide the desired function. A relay is what allows one circuit to control another - when something happens in one circuit it triggers a desired effect in another independent circuit.
Give some thought to what you have, what you want it to do, then draw it out to see what you might need.

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How about a joke?
Joke? Uh... what do you call a dead polar bear?
I don't know.
Anything you want, it can't hear you anymore.