Pathfinder Sources
I know Ultimate Combat and Inner Sea Combat, which came out recently, have some stuff about arenas in them.
Both talk a lot about the value of playing to the crowd. As a simple variant, consider giving the crowd's current favorite a +2 (or +4) morale bonus to hit and saves. Then let other people take that by showing off with perform or taking penalties to be showy.
Arena Building
If you're an old-timer like me, though, I'd suggest pulling out the Dark Sun Complete Gladiator's Handbook for ideas about arenas. My favorite was always the one in a natural bowl of obsidian, so there were razor-sharp protrusions and the arena itself became boiling hot over the course of the day. If a bout lasted too long, the combatants could end up being cooked alive.
Which brings up an important point: consider how to keep the pressure up. You want some mechanism to make sure the fight doesn't just last forever.
Delving into history, filling the arena with water and staging a naval battle is a classic. For the more magically inclined, that could be one part of the arena... perhaps with a fire, wind, and earth themed sections to go along with it. Or even an icy landscape at the far end of the arena.
If you do split up the arena into different areas, consider having the pressure apply faster to different areas. If it's heat, some areas will naturally be more dangerous than others. If the whole place is flooding with water, the lower areas will flood first and anyone there will be eaten by sharks.
I'm just assuming there are sharks. I'd release sharks in my arena.
Moving Goals
Perhaps instead of just "kill each other," there can be a more specific goal. Perhaps the combatant that is able to present the gem hidden in the heart of the dragon (kindly provided by the arena masters) to the judges at the far end of the field will be allowed free. Or items have to be collected that are scattered through the arena.
Taken together, you might have a few different methods of encouraging people to move around. Two is probably a good number, but I'd definitely include at least one way to keep people moving.
You can also start with something fairly simple, keeping the more complex option as a surprise in your back pocket. After all, the judges are always changing the rules at things like this.
Cheers!
Kinak
Are there any good links/adventures/ideas you all can share? I'm hoping for something more exotic than "I'll throw in some lions" which I've got so far 
