I just ordered some Elite Rings gymnastic training rings. I am really looking forward to using them: I will let you know what I think of them once I have worked with them for a while. They are currently being produced and I am waiting (im)patiently.
They are supposed to be great for suspended push ups, all types of pull-ups, muscle-ups, front/back levers, L-sits, etc.
I found them through the and older "Stuff we like" by Shugart.
I am not sure about your goals, but I would look at doing some gymnastics type movements: all you need is your bodyweight, but they are quite challenging.