Some cool suggestions so far; I like the Vermin though the CR seems quite terrifying. Here are my new thoughts...

Round 1: For Our First Trick: - 2 Balors. Easy, but its a chance for them to warm up and learn how to use their god-like powers. The arena is covered in lava, however, so positioning is going to be key.

Round 2: Nothing You Possess... - A giant, maze-like tower full of bizarre, lethal traps and tricks. A fellow group of adventurers will be competing to make it to the center for the "token" and victory.

Round 3: Choices, Choices - The party is forced to do things two-on-one, wrestling style, against a wide range of classic monsters: A drow ranger, a beholder, a dragon, some sort of high-level ghost, etc. They can tag people in and folks on the sidelines can buff and heal others on the outside or do sneaky stuff to help, and each party member can only be in the arena twice.

Round 4: A Bugs Life - 5 Devastation vermin. The goal is not to win; the goal is to last 10 turns. Bonus points for each vermin killed.

Round 5: Hercules! Hercules! - fight Hercules, who has some support in a wild and bizarre stadium setup for lots of chances at terrain use.

Round 6: The End of the Line - Zeus.

Round 7: BONUS: Eternal Reward - if the team somehow, someway beats Zeus, they get the reward of Godhood. Only one may be rewarded, however...so the team can either surrender, or fight to the death. Good times.

Interesting enough, or does it need more work?