Oh, there are endless possibilities! I hate to start off with something that could be perceived as dull or, if not dull, simply something that doesn’t need pointing out, but I do think that a home office would be an excellent choice for what to do with your spare room. Whether or not you work at home full-time, I think that most of us at least dedicate some of our home-time to work-related activities, and wouldn’t it just be nice to be able to separate those from what should be your relaxing non-work getaway? If not a home office, a library would be nice. With a nice chair you can sit in, and nice lighting? Or a home recording studio! You can put all of your instruments in the room and set it up with an oldish computer and an Mbox and put some carpets up on the walls. That would be great. Or OR, you can do, like, a home theater? With a projector and some nice chairs? Or maybe a room with a million pillows. I think it was on Destiny’s Child’s Cribs episode where it showed that they had one room that was only pillows and it looked so amazing. Any of those would be a great choice for what to do with your spare room.
Or, I suppose, you can take 8 months and build a 40,000 piece K’nex structure, with 450 feet of track, 21 different paths, 8 motors, 5 lifts, and “a one-of-a-kind computer-controlled crane,” “as well as two computer-controlled illuminated K’nex balls.” Or you can do that! It’s your spare room. You can pretty much do whatever you want with it.