Pull up bar
Pull up bar that can be extended/retracted to fit across any door frame (assuming its a fairly standard size door). You can either just rely on tension to hold it or screw through the plastic end caps into the doorframe for added strength. Strong enough to take average adult/teenager weight (not sure on the suggested limit). Supplied with fixings.
Works well but a word of warning, over time this puts a lot of pressure on the doorframe so we have seen cracks appearing on both the frame and surrounding wall.