Recreate a set from your favorite movie or TV show. Like the bridge from the Enterprise, or the bar from Cheers. Make something crazy and fun and impractical.

A stripper pole, doesnt need drills,(for working out or friends whatever lol) , a mini bar, lil waterfall, use vilnyl wall decals that peel on and off walls theyre very in style and cheap you can make your own too. Theyre bi sexual too...you know like male/female you know what i mean

Oh, I forgot to mention, the panels are available at Home Depot for only about ten bucks each, and they can be cut with a utility knife and a cutting guide. You'd need to use four panels for each of the fifteen foot walls, and cut the fourth panel down from four, to three feet wide. Try one panel first and see how tightly it will wedge between the floor and ceiling and use this as reference. If it stays up without bowing in the middle, you won't need to trim it at all.

Turn it into a man-cave! Install a large video screen, Video projector, with a surround sound system and a wet or dry bar. Put up lots of nasty pictures of sexy chicks, team logos, posters, etc. to completely cover the walls. Oh, for the noise reduction, so that your neighbors don't hear anything, install 4X8 foot sound deadening boards to the walls first, then cover them up with your "graffiti". Furnish the room with bean bag seats and used mattresses, covered with some kind of washable cloth materials. It would take about 16 of the sound boards, which you could join together with long, flat 1x4's on the back side and force them against each wall, using the thickness of whatever carpeting there is and the ceiling. The distance from the top of the carpet edges to the ceiling would be just short enough for the 8ft panels to wedge into, thus keeping them in place without nails or any other damage to the walls. As you go from one wall to the next size each panel so that it will fit tightly against the surface of the previous edge, and so on.

Put any "non-sleeping stuff" that is in your bedroom, and any "non-living stuff" that is in your living room, into your spare room ~

Books and magazines; desk top computer and peripherals; games machines; tools and "projects"; sports and camping gear; park your bicycle/skateboard/kayak in there.. ; anything else that doesn't quit "belong"...

~ and you no longer have a "spare" room. And a tidier and neater apartment..

Put in to hammocks, a HUGE ball pit under, and a bunch of fish, ( or maybe make it a library but the first one is funner) :)

I've got an extra room I may turn into a personal arcade, I've already got a coin-operated Road Blasters and Buck Rogers.

I would turn it into a photo studio. But you're probably not into that. If you're into it, you could make a sports bar/poker room, or a mirror on the ceiling, and well...you know the rest.

laser tag!

Replace the door with book shelves, tilt one book to release the catch...

Behind it, add sound insulation and you can get up to all sorts.

_{^{With all sorts...}}

Massive games room. Pool table, darts board and a mini bar. Whack a TV on the wall and hook and xbox up to it. Beanbags everywhere.

Im going to have to say bowling alley or just open a restaurant franchise in there, that way you dont have to go out to eat

Have a half-door, the ones where you can open the top half, and keep the lower-half closed, and make a ball pit!

paint the walls blue or green, mix a bit of white in with it for the ceiling (ceilings look darker than walls), get some carpet offcuts of a similar colour for the floor, and BAM green screen studio for video fun. or just build a holodeck... whichever.

Replace the door with the same color of the walls and then make a massive hidden game room or excersie room.