I just finished a campaign where the party performed in many arena matches. Depending on what you want to do here are a few ideas.
Skill Based: We had a halfling with high balance and tumble abilities. So her challenge was to walk a tight rope over a vat of pirana. The vat was surrounded by hecklers who threw rocks to try to disrupt the balancing act.
Lumberjack style contests. The fighters performed various feats of strength and skill, but not in direct combat with each other.
Fighting centric: One combat field had a series of rotating blades and spikes under the sand. The field would randomly activate and the party had to fight other creatures at the same time.
Same idea, but with random spell fields. One 10x10 area would create the bull's strength effect, another would create the concecrate effect, etc. It randomly buffed or weakened the combatants.
The one I could not get anyone to take, fighting in an arena flooded to hip level. The party would not even fight low level orcs in such an environment...
