Prizes for a Convention?

I am running three separate Castles & Crusades sessions at this falls U-Con gaming convention in Ann Arbor Michigan. I want to give prizes to the player in each session with the most X at the end of the session. The questions I have are,

1) What should X be? Meaning, the most what? The most creatures killed? The most "cool" points (a random name for some token I would give to a player when they do something cool)? Should I have the players vote on who was the best roleplayer? The best hacker-n-slasher?

2) Once I've determined what X is, I need to determine what the prize will be. I originally thought that since I was going with an "old-school" theme I could give away some new GameScience Zocci dice. Maybe one complete set per top player, which means I buy three sets, or about 20-30$ depending on where I get them from. Does anyone have any better suggestions?