Hiya! My party's home base city currently has a fighting pit/gladiatorial arena being built as a tourism pull. 

Have any of you dabbled in arena fighting one shots?

What worked? What didn't? What are some great ideas you've had on this topics you'd like to share?

I have always done arenas as straight up fights.

about 12 years ago I did a more Roman games style, creating several different events so that different classes could compete. So, like performance for bards, hide and seek for rogues, mage duels for wizards, that kind of thing.

it was the stuff other than the straight up combat (even if it included a naval battle) that got my players excited. But they are weir, lol.

If you want something that is simple, Xanathar's has a section on Pit Fighting that is quick and really fun (link below).  I like to modify this to not always be Athletics, Acrobatics, Constitution as the three checks.  Against a Rogue opponent, make one Perception to see them stealthed.  Against a Cleric opponent, make one Religion to identify their holy symbol and know their weakness or something I dunno.  Stuff like that

https://www.dndbeyond.com/sources/xgte/downtime-revisited#PitFighting

Straight up PVP battles are another option that is super fun but also rather time consuming and usually frustrating for the loser because DnD PVP is not remotely balanced at all. 

The important thing for these kind of battles is to still have dynamic battlegrounds. Have some pillars to climb and/or hide behind, maybe a moat running through the middle... ideally even a few hazards on the stage... maybe a big boulder that comes rolling through randomly. The important part is to make it something other than a big, flat, empty circle that doesn't allow half the classes in the game to use their unique features in interesting ways. I think it also helps to mix things up if there's occasionally a secondary goal in the battles... like maybe there are items that need to be collected strewn across the battlefield, or there's a king of the hill challenge where one team has to occupy a vulnerable position for two full rounds to win. And be sure to establish that there's a Loser's Bracket running as well... that way if the players do lose a challenge it doesn't just end the whole thing and waste your time prepping the rest of the tournament... it can also be a fun excuse to bring back a particularly memorable enemy from earlier in the tournament.

It can also be fun to use this as an excuse to build some opposing parties... I always have fun doing themed groups. I recall doing one contest where the players were battling against a group that was themed around GI Joe, and a second group that was a bunch of just rich snobs for a classic "slobs vs snobs" rivalry that all the players instantly latched onto.

I have run a pit-fight as a 1v1 combat. The monk went in against the pit fighter.

I made it a "Ropes" match, which saw loads of ropes around the room which dropped different things into the pit. The pit fighter was gettign beaten up by the monk until one of the other players untied a rope which dropped oil on them both, and the pit fighter laughed and said "let's see you run around now boy!" before landing 3 solid hits on him.

The fight ended when they were both flattened by a chandelier, and the pit fighter was knocked out (0hp), the monk had 3hp left! This was at level 8, so very close!