There's an Iron Gym rig at walmart for 15 bucks, nothing more than a bar that you can screw to a door frame/halway, the bar sits between two mounting brackets and you can remove the bar and leave the brackets whenever it gets in the way, comes with four brackets so you can mount em lower and use the bar to hook your feet under for situps. I own this one, works pretty well, tho don't put it in too wide of a hallway because it's telescoping, and the further it stretches out the weaker it is.
If you don't want to go drilling holes in your wall then there's a rig that you can hang in your door frame for like 25-30 bucks, they sell em at walmart too. does the sit up thing also.