[Kale]

A less WMD style plot might have the Big Bad finding a cache of pre-apoc armored vehicles. Maybe his 'scientists' are trying to get them working again and/or make fuel for them. When everyone is armed with crossbows and makeshift muskets if you can field a few tanks or even armed APCs you will crush pretty much everyone.
The PCs can break into the villain's lair, destroy his fuel production facilities, free/kill/kidnap his science advisor(s), and drive off into the sunset with a single captured APC or tank after blowing up the vehicle hanger. The PCs get a cool new toy as reward for completing the mission. It also sets the tone for future adventures, as the PCs struggle to keep their new toy fueled and repaired...