One of the reasons why calisthenics is so amazing as a training method is that it doesn't require almost any gear. However, having training equpiment allows you to do much more exercises and makes your workouts far more comprehensive. I have collected a lot of equipment throughout the years and I'd like to show it to you, and share my thoughts on each piece. This is simply the best piece of equipment you can get, as far as I'm concerned. I'm not going to repeat myself, though, I made a whole video about the rings, which I suggest you to see if you haven't.
Anyway, I bought my rings a long time ago, and they still are my favorites, that's why I put them first on the list. On some level, they substitute for parallel bars, push up bars, ab wheel, pull up bar, and probably a lot more. If you have to choose one thing you'll get, that's the way to go. Push up bars are pretty much self-explanatory - they are used to do push-ups. They let you go deeper, and thus, increase the range of motion. They also alternate the grip, and supporting your body this way puts less stress on your wrists.
You can also do different push up-like exercises and handstands, although I personally find these bars not very convenient for handstand workouts - the bars are too narrow to be comfortable. Sometimes, there's also a stability issue, especially if you're a beginner. Parallel bars - these are actually very similar to push up bars, but are superior to them. I made them myself and have fixed all the issues the push up bars have had.
They have wider and longer bars and are more stable. This makes them more suitable for handstand training. I recommend making them on your own, as its easy, cheap and the final product is often better than the store-bought one. It is probably the most...
See more here: https://www.youtube.com/watch?v=raVlC0P0sDY