I've started doing pull ups. Not a lot, seriously, not a lot, like 3 a day. However I've been impressed at how well it works. In little over a week I've gone from hanging off the bar, to being able to do an actual chin up. I don't feel sore and I don't even break a sweat, but the progress is steady and rewarding. What I like best is that no matter how busy I am, I can always spare 2 minutes a day for it.