Colosseum encounter help

So I have just started running a new campaign. To start I had each of my players submit a reason why they would be in the dungeon of the capital city of the area. The city throws an annual festival which includes colosseum games. The grand prize for winning these games is, you guessed it, freedom. There is more to this freedom that my players are unaware of at the moment. If they succeed they will be given two options. The first is actual freedom, but would include banishment from the capitol and surrounding lands (this is a homebrew world and they would be being banished from what are considered the "civilzed" lands). The second option is to earn their way back into the good graces of capitol. This would involve mostly dirty work to start but as they accomplish more tasks, hopefully without major incident, they would be given more and more freedom as well as some nice government sanction loot (along with anything they might find along the way).

I don't know if the backstory is really relevant here, but I felt like giving it anyway. Now to the actual problem. As they start, their characters have been through many rounds of the arena and have been successful. They essentailly know the ins and outs of what might happen. I have put them through a round of actual play. It involved them fighting a group of goblins on fair terms as well as an ettin they brought into the battlefield by accidentally triggering a trap. They handled this encounter fairly well and got away with mostly just a few scraps and bruises (the sorcerer played a key role by directing the ettin at the goblin force through clever use of minor illusion). They all seemed to really enjoy this encounter. Now I am letting them take a short rest as they prepare for round 2, the final round. I figure this round needs to be something major. I keep trying to think of something besides a big monster or just another straight fight. I want them to have to think. What I do not want is to make up some crazy puzzle with a single solution. My party is a fairly creative bunch so I want to throw something at them they can have fun with but should still be a good challenge. An important piece of information would probably be that my party consists of 4 level 3 characters. A ranger, wizard, paladin, and sorcerer.

Ideas I have considered so far are:

• Each character is moved to one of the 4 entrances of the arena (it is a large octagon) and enter the battlefield with a doppelganger under the guise of another party member. They would have to try and figure out who is who, as would the doppelgangers, who have been weakened from their time in the dungeon. Obviously 4 levels 3's vs 4 CR 2 enemies, especially a doppelganger would end in tragedy so they would not be at full strength.

• Characters would enter the arena chained together in sets of 2 against some throw happy ogres. There are pits in the arena. I would allow each set of characters to act on the same initiative. Also they could break or remove the chains if they think of it and get creative.

• Something with a key and some gelatinous cubes, I dunno. I just like gelatinous cubes.

Here is the map I am using which I threw together at pyromancers.com: [Map!] (http://i.imgur.com/C6tDW6a.png) Obviously the 4 big holes are in fact big holes. These are pits that pretty much guarantee death if you fall in one. They can be covered depending on the event. The north, south east and west have big doors for the contestants to come out of. The grey squares throughout the arena are potential trap squares. They can be enabled and disabled for whatever is needed. The red triangles along the diagonal walls are wall spikes.

This ended up being a much longer post than I meant it to be but whatever! If anyone has some fun ideas for an arena encounter please share. Assuming my party does not choose banishment, this will probably not be the last they see of this arena. Thank you for your time if you made it this far, I tend to ramble.