Hi everyone.  I just started a new campaign with two friends and the first adventure ended with them being branded and sent to a fighting arena.  I thought writing that sort of adventure would be easy – throw some exotic or mundane monsters at them and let them fight it out as my players love combat, perfect for Christmas!  But I was wondering if there is something better.

 I know this is a very common idea.  Has anyone run a gladiator type adventure?  Any good ideas?  I thought perhaps that the overseer might train them, giving them bonuses in stats or abilities, as they want to make them better fighters, hence bringing in more spectators.  Perhaps someone in the crowd assists in some way if they can win them over.

 So any good ideas for enemies, training and/or patrons?  Thanks!

Since the premise is so simple, you can use this to your advantage and give players more roleplay opportunities. Create friends, allies, and most importantly, rival NPCs for the players to interact with.

Your players might get bored if it's just monster after monster, but if there's an NPC they love to hate, competing with them, suddenly their emotional investment transforms the campaign into something more exciting. 

Create a side game of marketing the battles the character(s) are in. Know what's better than fighting in an arena? Fighting in a PACKED arena!

This would create some social opportunities and could lead to higher profits, etc.

AND

Counter hype from other parties who are betting against the character.

When I’ve done coliseum fights, I always make sure to change up the terrain, as a big open field is kind of boring. 
Add in some zones that are periodically dangerous, like a grate where fire spits out every other round. Or some ramps and platforms to allow for height differences. Areas of difficult terrain. Some walls for cover. That sort of thing. 

And there’s the old, flood the place. Water is 6-8 feet deep so they swim if they’re in it. Add in a few floating platforms/rafts for them and enemies. Maybe some sharks or alligators in the water that will attack either team for a little element of chaos.

Simple toe-to-toe battles on flat open ground can be a little underwhelming after a while (D&D just isn't made for that) so it would help to have some "set dressing" that the players can use in the battles.  There could be ramps and platforms, with special weapons hanging above for players to use, while a large monster is rampaging through and smashing them down.  There could be magic towers shooting fire bolts, and by making them to target you (by being the closest creature to it?) you can direct its attacks in the direction of the monster... if you dodge well.  it could be as simple as the arena's weapons being poor quality, so they keep breaking and the players have to run around getting new weapons scattered around the arena, instead of just standing in one spot hacking at the creature.

If you want to get creative, you could give each monster a unique ability, something that works outside of the normal initiative/action mechanics, for the players to interact with (or at least react to) - kind of like "lair actions," but with some flexibility for players to react on the spot too.  Maybe the monster "telegraphs" a special attack it's going to do, and the PCs need to choose how they try to defend/dodge it.  Is it preparing to breathe fire, pounce, or swing its tail?  If you guess right based on the DM's narration, it automatically misses you.  

Another thing you could do is involve the spectators a little bit and create some interactions (or even mini-games) to participate in.  If you can gain the favor of the crowd, by grand-standing during the battle, maybe you get bardic inspiration, or something similar?  If you've become a sort of "crowd favorite" then maybe a local aristocrat asks you to wear the colors of his house when you fight, offering something of value if you agree... and actually win your fights, of course.  

(edit: fixed spelling)

The above have covered good suggestions on making individual encounters in the arena more interesting, but you might also want to consider where the story will go from here? Are they going to be stuck in the arena until they complete a certain number of encounters? Do they need to win over the audience to gain their freedom? Is the campaign entirely going to revolve around them making the arena successful? Are they going to uncover a larger evil plot involving the arena - e.g. gladiators being hired by criminals / as assassins, corrupt betting / fixed matches, some weird evil group involved in procuring the monsters to be fought?

Because just a series of combats in an arena will only hold their attention for so long, eventually it will get repetitive / boring.

These are good ideas.

The "pit boss" told the players they need to win three battles to earn their freedom.  So there is a definite end point.  The next adventure will either be getting revenge on the NPC who betrayed them and/or a clever heist job from a patron they meet and/or impress while fighting.  

I agree that endless fights in a round area with a sand floor are terribly boring.  They had that in the Baldur's Gate II expansion pack and it was terrible.