Apply a negative level after each day adventuring. Set the DC so that it is "Determined by the strength of your soul" or something, so the DC scales. Make it so it targets the weakest of their saves, not necessarily Fort. Set the DC so they usually fail about once every 4 days. That should be just a bit slower than their level gain, so if they slow down at all they start to lose strength. Change the numbers to fit your idea, but as long as it's a short-term campaign they shouldn't gain or lose too much power, but they'll feel epic when they beat the BBEG and can actually take a rest.