Hmm, maybe some sort of penalty for cancelling a rental early towards the end of the season?

I rent out a few cards myself. I don't adjust the prices because I'm inherently lazy, but honestly, the demand for rentals in the last couple of days of a season seems insatiable... it's so high that, yeah, as you said, I'm not sure much can be done that doesn't accidentally affect the rest of the season (ie, people might keep their rental prices super high just to cash in for end of season).