Most people who run their first Rock Paper Scissors tournament say the same thing afterward: it was easier than they expected and the crowd was louder than they expected. That combination is why people keep doing it. A well-run RPS event has a kind of energy that's hard to explain until you see it. Strangers get genuinely invested in a match that takes twenty seconds.
The format question is the first one worth thinking through. Single-elimination moves fast and builds tension because every match matters immediately. Round-robin is better if you want everyone to get more playing time and the outcome to feel earned over a series of matches rather than decided in a single unlucky throw. For a first event, single-elimination with a best-of-three or best-of-five format keeps things moving and finishes in a reasonable time.
The bracket itself doesn't need to be complicated. Eight or sixteen players is the sweet spot for a casual event. Eight players means roughly six total matches across three rounds, which takes under an hour including delays. Sixteen is still manageable and gives you a bigger pool of participants. Seeding isn't necessary unless you know the skill levels in advance. For most informal tournaments, a random draw is perfectly fine and sometimes more fun.
Space and timing matter more than people assume. You want enough quiet that players can hear the cadence count clearly, and you want a table or hard surface for a referee to stand at. Outdoor events work but wind and ambient noise can create disputes over what was thrown. The actual throw has to be simultaneous on the count of three. Use "rock, paper, scissors, shoot" if that's the regional custom. Either way, you need someone whose job is to watch both hands at the same time and make the call when it's close.
The thing that kills a tournament isn't cheating or disputes. It's downtime. Keep rounds moving, have the bracket visible so people know what's coming, and don't let winners sit idle for long. If you're running a small event, someone with a phone and a photo of the bracket is more than enough to track results in real time.
You don't need prizes to make it worth playing. But if you want to add stakes, a small prize for the winner and something for everyone who participates tends to keep the room invested through later rounds. People stay for the drama when they feel like the event was run fairly and the competition was real.