I have three rooms which have all evolved into junk collections since the boys left home. The eldest, who is married and lives in the US, has left some stuff in boxes, but I made him promise to throw everything away that he really wouldn't need again. I wasn't going to provide storage for five years for something if he was only going to come back and chuck it away.
Middle one has completely cleared his room and that is a useful spare room, but I keep the ironing in there. Youngest still hasn't really done his room out at all, although I ask him to every time he comes home. He is still at university, so I wouldn't want him to think we were chucking him out.
To answer the original question - you could turn your "spare" room into a gym, a private cinema, or a reading room, or a library. The most likely is ironing and sewing, though.
I am in the process of gathering all the bedlinen, bedding and towels together to keep in one single place all together in one of their wardrobes, but much sorting will still be necessary.
And I agree with what others have said. If he's a Uni, he's not really left home. That is surely not until they finally move in with a girlfriend or start a family.