More information could be helpful. You say that you are single with no kids. Is that the way it's going to stay? You don't often have friends over, but will that change? Perhaps with a new home you are planning on doing some more socializing? What are the dimensions of the room? Do you have a basement that might be allocated to some of your ideas? Do you still have a 'guest room' aside from this for, well, guests?
So, we're talking one bedroom here, right? What's that about 12' 12' at most? If you stick an air hockey table or foosball in there, either one would really eat up space, almost filling the room. A dart board doesn't take up much space.
Do you do a lot of reading? If so, maybe you make it a study or library. A couple club chairs with wall to wall bookshelves.
Are you into cigars? You could make it a nice smoke room / humidor. Only the single guys could get away with that!
How about a bar area? Bar, stools, kegerator, pub table, TVs. You could make with a Tiki theme, sports theme, rock and roll theme, movie theme, etc.
Do you like animals? Depending on your local laws, you could make it a dedicated room/cage for an exotic animal, like a monkey.