Originally Posted by Steven Wingo
Consider a TRX. There are several different versions. They are made to be portable and have different options for hooking them up to things like doors, tree branches, roof trusses, and so on. They would allow you to do ring rows and a host of other body weight exercises such as scaled push ups. It is a great option if you need to scale body weight movements and don't have access to a full gym full of equipment like rings, boxes, etc.
TRX are nice, but I would just go with a ring set. Not much that TRX does that rings can't, they're cheaper, and once you're stronger, the ring stuff is the "real" stuff. If your rack has an over head crossbar you can begin to do ring pullups as well.