If you want to control multiple devices, you'll probably find it easier to work with the parallel port, because that gives you 8 independent IO lines.
Garage door opener shouldn't be hard. All you need to do is take apart a garage door remote and have it controlled by a transistor or something.
Trunk button? To pop the trunk? Aren't those generally done with cables? That might be considerably more difficult. A hefty solonoid could pull the latch, but positioning it unobtrusively would probably be hard. On the other hand, if yours is controlled with a button that closes a circuit, then it'd be no different than the garage door opener.