A few ideas:
Idea 1:
Become a master of teleportation magic and become familiar with the elemental planes. Open portal to your favorite elemental plane above every seat of power and powerful army in the campaign setting, one by one. Flooding a city full of living fire = win. (For bonus points, open a portal to the opposing element afterwards and watch the elementals have it out with each other.)
Idea 2:
Find a really good charm spell. Charm every head of state to each normally, but obey your orders. Charm anyone who suspects that you have charmed anyone important. Be best friends forever.
Idea 3:
Create false big bad evil guys for your party to defeat. Create a few atrocities here and there to incite the populace, but never leave any witnesses. Find some half-brained moron, hand him a few dozen undead, and stick him in some shack on a mountain shaped like a skull. Tell the local leaders you have tracked down the source of the evil deeds, ride off into the sunset, and come back with the poor moron's head on a stick. Voila! Instant heroes! Rinse, wash, and repeat, until the populace is certain that only you and your merry band of avengers can protect them.
Idea 4:
Create a magic item that slowly drains the intelligence of those around it over a period of time. Enchant to make it interesting enough for people to hang around it(moving pictures, music, lewd sounds, whatever it takes). Mass produce enough of these magic items to stick one in every hut, hovel, and hostel. Wait several months, and WHAM!, you're the smartest person in the land. Tell them that running a kingdom is really hard, and if they wanted, you'd take on the burden so they could watch the pretty pictures, or drool, or whatever it is they want to do.
More upon request.