If I were you, I would sell them as fast as I can. SuperMan is a well established trademark known the world over. Any development would mean that you are trying to profit from someone elses trademark. Not good. You'll most likely start getting cease and desist emails and letters. Or maybe you'll get lucky and the no one will bother you. Is it worth the risk? I don't think so.
Park them or try selling them to a company that has the right to use the SuperMan trademark, e.g. whichever toy company will be doing the merchandising.
However, in the regular retail or wholesale domain market, I'd say reg fee.
As always, IMO.
AmCy