You ever get stuck in a rut and not know how to get out of it? That catch phrase, "Think outside of the box", will usually come to mind. The best way to get out of the box is to use a portal to someplace else, ANYPLACE else. You don't have to change the box walls, interior, anything. Just make a gateway to someplace else that frees you from the constraints you have placed upon yourself in your current world. Remember portals can work both ways, so let some crazy in to your campaign or let your campaign in to some crazy!

Some suggestions:

Portal to the Abyss

Who doesn't like killing demons? There are tons of great things needing deadening in the Abyss that even the most Lawful Good player would kill without a second thought. Spice up your play with a random vortex of power that dumps denizens of hell on unsuspecting PCs. Nothing funnier than waking up to find a Quasit on your chest!

Portal to the Planes

A vacation to another plane of existence always keeps PCs on their toes! Send them to Sigil for a day or two on a limited quest to find some artifact. Throw your PCs in to an elemental plane where some scrying NPC just happens to be waiting for them with gear to "loan" them so they can explore that plane and complete some quest they may not even have known they were on.

Portal to the Bottom Of The Ocean

My favorite. How did that crazy Kraken get in to the pond? Some long dead magic user openned a portal to the wrong place and lost control of it. Or maybe that magic user openned a portal and died before he could close it again. Or maybe that magic user openned the portal to let loose chaos on the land from the sea! Or maybe...

You get the idea. Anchor one end of a portal in your campaign and let the other end go wild!