Planning: Come up with a clear structure of games or challenges first. It would help to draw from existing gameshow formats — just replace trivia answers with riddles or locked doors, athletic challenges with combat scenarios, or cooking competitions with alchemical concoctions.

Player Involvement: Make sure to keep everyone involved during the PVE part. Try to include challenges that cater to the strength of each class. For instance, wisdom-based challenges for clerics and druids, strength-based challenges for fighters and barbarians, and so on.

Currency System: A points or gold system would be an interesting mechanic. The more challenges the players complete, the more points they get, and these can then be exchanged in your item shop for equipment or buff that will prove useful in the final battle.

The Item Shop: Before the battle, do give players opportunities to equip themselves ideally. Include consumables, equipment, magic items or even traps that characters can use during the battle royal.

Non-combat Challenges: Not everything should be about combat. Set up contests of wit, agility, or creativity. Mystery solving, obstacle courses, crafting challenges, or role-playing scenarios might be an interesting change.

Bombshells: You should add a surprising twist in the middle to keep the situation exciting and keep participants on their toes. For instance, switch up challenges, enforce a surprise task, or an unexpected NPC.

Balance: Take care to balance your challenges. They should start easier and get progressively more difficult, but not impossible. Remember, the aim is for the players to have fun.

Time Constraints: This will put pressure on the players and minimize lengthy discussions and planning, intensifying the game’s drama.

PVP Fairness: If you’re ending with a battle royal, make sure to address players’ concerns about PvP. Reiterate that it’s all part of the game, ensure you’ve got rules in place to deal with PvP scenarios, and remind everyone to respect each other’s character choices.

Encourage Roleplay: Player interaction is at the heart of a gameshow oneshot. Reward clever roleplay or thoughtful in-character decision-making.

Reward: Offer a worthwhile prize at the end, such as a magic item, reputation, or even a wish. This will motivate the players to play competitively.

Tie-Ins: If this oneshot is part of a larger campaign, consider ways to tie outcomes back into the main campaign story. Remember, the main goal is to entertain the players, ensure everyone has a chance to shine, and create a memorable gaming session.#

2 Minute Tabletop has a great module you can run. I just ran this with my family on vacation when we were looking for something to do in the evenings. I extended it a lot with some fetch quests before the tournament, and everyone had a good time learning their characters/practicing their mechanics.

Relevant links: https://2minutetabletop.com/greybanner-arena-rules-activities/

https://2minutetabletop.com/greybanner-arena-tournament-rules-roster/

You could totally do something TaskMaster style.

I ran a battle royale style one shot for a big group recently, based on the Battle Royale module from DMGuild.

Because it was such a large group (11 players) I made a bunch of compromises:

• initial 2.5hrs was team based, and sold to the players (in character) as if they could achieve victory together
• during this period, each area/activity they wanted to achieve was run as a skill challenge
• 6 success before 4 failures to succeed on the challenge, DC changed depending on the activity
• expending a short/long rest resource or spell slot could get advantage on the roll.
• I kept it to 2 areas where they could acquire loot, 2 random travel events and a final longer (8/4) section of the final circle
• in the PVP section, the in-game game master announces they must fight on to the death
• again with this many players I needed to try and keep the game moving, I used NPCs and lair events to keep from players from hiding in the corner too long
• as rounds went on I added another d6 to the lair action damage at the top of the round. Eventually I started upping the DC on the dex checks just to keep things moving as well.

As the other comment said, it was really important to be upfront with the players, out of game, about the challenges of PVP. It was unavoidable for players to be sitting around waiting for the combat to finish with little to do.

I also found delegating some tasks made it easier for me to keep a handle on such a large group:

• timekeeper, trying to keep us to the limits I set at the beginning
• loot giver, I’d announce how many/type of chest they found which corresponded to how many times they could roll on the loot table, and on which one. One IRL player did this for me and distributed as best he could