My workout buddy has some gym rings he bought for $85 or so, that we have been using during workouts.  I started thinking, I need some of those too.  But I didn't want to shell out the big bucks, so I typed into Google, "how do you make gymnastics rings?".  Turns out you can make them pretty cheaply with some cinch straps from Harbor Freight, some 1" grey Schedule 40 PVC conduit, a couple of eyebolts, and some sand.  

Here is the Instructable that I followed:  http://www.instructables.com/id/How-to-make-GymnasticsFitness-Rings-from-PVC-Cond/  You cut the PVC conduit to length, fill with sand, heat in the oven, and bend into rings.  (I used the inside of a bucket to shape mine.)  They cool fast, hold shape, and are strong once cooled.

They are cheap, easy to make, and sturdy.  I bought some snap links to hook between the eyebolts in my garage ceiling joists and the straps for the rings, so I can put them up and take them down quickly.  The only thing permanent I added to the garage were two stainless steel eye bolts into the ceiling.  If you are looking for an easy project to add to your workout equipment, check into this.  I probably have less than $30 into all of it, and it only took me about 30 minutes start to finish once I had bought the materials.