We ran the first ever FRC event in our state as an off-season, and we did a 26-hour endurance event, so we learned a thing or two about "non-traditional event" logistics.
First thing, as your first event, I would restrict it to 24 teams. This is the minimum you need for a competition, and it keeps your numbers low so you can work out the "bumps in the road" at first.
Second, I would go to some other off-seasons, have your students and mentors volunteer, and have them pay attention to the small details that make these events run smoothly. Ask them about how they handle registration, volunteer coordination, etc. The little things are what can make or break an event. Things like having printed schedules for every team (the FMS system can do that for you), having a centrally-located screen to display real-time ranking information (FMS video output), who is running the A/V (FTA can usually help with the FIRST side of the A/V system), floor markings for robot and human directional travel, signs for where things are, concessions, etc. Keep an eye out on all of those things and have your students and mentors write stuff down.
This is a document provided by the guys that run IRI. There is a lot of helpful information there. There is also the
list of volunteer positions for an official event and
FIRST's web-page about hosting off-season events.
I'm not sure about the accuracy on all of those pages, though, because we got our field from AndyMark, which included things that FIRST says are not included. I'm guessing you can get a field from either FIRST or AndyMark? Because ours came with carpet, game pieces, etc. We also got an FTA staff from AndyMark.