Could it be...
Cole lying in bed... lifeless?
Cole lying in bed... with Lu Ann?
Cole lying in bed... with Lu Ann and some guy in clown makeup...?
Cole eating the world's largest ice cream sundae, and he didn't tell Marty about it, that pig?!
I know the whole lifeless (or almost-lifeless) body thing is the most likely, but I'm really rooting for the giant ice cream sundae. What do you guys think Marty sees in Cole's room?