Rings are not expensive, around 30$ the pair. You just need somewhere to hang them from (garage rafters, tree, pullup bar, ceiling mounts, park, etc), it's better if it's something high, as it will allow you to do more things, but even a simple doorway pullup bar is enough to do the basics like rows, pullups, dips, and pushups. It also has the benefit of not taking a lot of space (wich I personally consider a huge pro)

On them you can do anything from rows, pullups, dips, muscle ups, ring pushups, toes to bar, and L-Sits, to planches, levers, iron crosses, malteses, etc

These should do (any other brand will too).

Looks like they're very similar to the ones I have. I've been happy with them.