A brief brain dump I've been putting together throughout the day (in an inconsistent and non-proof read way):
The main problem I ran into was that it wasn't a tournament.
In our case, things were made a lot easier by the fact that Foundry didn't charge anything and were able to provide the entire venue, tables, scenery, toilets and the basics at no cost at all. But, being free was its own two edged sword:
Because entrants (gamers) didn't need to pay to get in (as I think is typical of most normal gaming days at clubs), the commitment to appear on the day was not the same. Many people who were making the rigth noises just didn't appear, including events organisers. By the same token, lots of desire was expressed for certain events, but on the day, none of those players pitched up. If you can find a solution to this problem, I think you'll have a much easier time predicting what will happen on the day.
Things we sort of discussed but didn't really do anything about:
1. The painting competition (golden gobbo)
2. Trading/selling
3. Art and figure displays
In our case, #3 worked out the best without much guidance, with a vague nod to #1. I got the impression most people would have wanted more formality around #2, presumably in the shape of a bring-and-buy or an official trade stand so that people knew when and where to trade.
Again, because its not a tournament, I didn't feel it would be right to try and set any army or scenario rules (like points sizes) - nor did we have a means of matching opponents to each other. I think in a tournament, its easy because the attendee can just pitch up and be told what to do and where to go, whilst pretty much knowing that things will be okay because everyone else is obeying the same rules as him.
We didn't have any 'fair' (an interesting concept in oldhammer generally, but lets use it in comparison to the tournament approach) way to match opponents to each other, so we decided to leave it to the players to organise on a thread on the forum.
I believe that all the games that were pre-arranged were much more successful and interesting than the ones that spontaneously happened - so, if you can compel your attendees to do this, or you find a system that provides guidance without mandating behaviour, you're on to a winner.
On this game arranging thread, I got the impression that generally, people were afraid to set scenarios or ask for the type of game they wanted and I suspect this was as a result of fear of judgement from the community. I think this was also because no-one really knew what to expect, so the idea was to aim low. Laziness probably had a part to play as well. In the end, the few scenarios that were proposed ended up becoming the large 'event' games on the day. As a result of a lot of support by both the players and having GM's, I think these games looked like the most fun of all.
That said, I think that many of the players would have liked to participate in one big multi-player game on one of the days and play a smaller game (or at least, a different game) on the other day, so bear this in mind with organising multi-player epic games.
In terms of the large games, I would strongly recommend not having more than two player phases in a turn - having a three, four or five player turn sequence is hard to manage and takes a huge amount of time. This does suggest a nominal A vs. B style of game - but because you're at a show, I think that's a bit better.
