The myth that you cannot gain muscle mass or significant strength from body weight training is bull-shit. Just look at guys like hannibal for king or barstarz on youtube and you will agree. Sure their legs may be a little under developed but nothing that some deadlifts or squats can’t fix. Anyway you can go extremely far with body weight training. You just need the right equipment and a lot of variety. Ya. A school yard is by far the easiest and cheapest way to get the right equipment and variety.
The main exercises are of course chin-ups, dips, handstands and push-ups. These can be done weighted or with a rainbows worth of variety. The number of variations of body weight exercises are literally limited by your imagination. There are an infinite number of variations if you think about it. Each one can transition to another and body segments can be altered to make it easier or more difficult. Impressive skills that a person can work toward are the muscle-up, one arm chin up, free standing hand stand, superman dips or really whatever your heart desires. I believe that the best upper body training always incorporates core stability at some level. Body weight exercises accomplish this.