[Rich Wong]

A single-day FLL event for fundraising and to introduction non-FIRST schools to FIRST LEGO is a great idea.

With 12 full FLL kits, you can support 12 possible teams of 4.
Because of time limits, the event must flow quickly to maintain student interest. The students participating in the event may have little or no experience with the RCX programming, therefore I would avoid programming completed.

You can create 5 different generic programs for all the RCX unit. Each programmed pattern will perform different FLL tasks. The students will need to study these patterns and match them to the field missions.
All the RCX units will be pre-programming with these 5 patterns before the start of the competition.

Have each team construct the LEGO robot around these pre-programmed PCX. The performance of the robot will be determined by the design of the robot and the program they elect to run during the matches. That is enough variety to create a fun and easy competition that will quickly introduce new students to FLL competitions.

If you can afford to run this type of event once of month, you will build a large group of followers who will want to start their own FLL team. All participants can receive a certificate or and actual FLL medallion (depending on your budget).

Trophies are always treasured and creates more excitement and competitive event. There are resources in usfirst.org under FIRST Jr. Robotics link.
Good Luck!