We have something called the perfect pull up bar (or something like that). It cost about $20 from a local sporting goods store, but I have also seen them online, including on Amazon.
I like it because I have it installed at a height DD can reach by herself, and I can remove it as necessary because it is only the frame screwed into the doorframe. The bar itself hooks on to that framework.
On the down side, the screws that came with it were not long enough to adequately secure it to the door frame, so I switched them out for longer ones. Also, our box lacked installation instructions, so I had to look them up on line.
Otherwise we are happy with it. I hung on it before DD got on it. I'm about 4x her weight. I figured that if I didn't bring the thing down, she'd be ok.
