You should have copies of the various BGA leaflets to give out to those you teach. It is always hard to know how many to bring, but you would be doing well to teach 50 people in a day. The cartoon booklet is designed for children, but as it is simple adults like it too.
For passing trade and casual enquiries the trifold leaflet is the cheapest to give out, so a larger stock of these should be brought.
All leaflets can be obtained from the BGA via the Secretary.
Also you need a few things to show people. A beginners books is useful, for instance "Teach Yourself Go", so that people can know they can study Go if serious about it. If it is an anime convention then a display copy of "Hikaru No go" is a must. Sample journals are also available from the BGA, for display purposes or, if sufficient are available, for giving to people interested in joining.
Make sure you know the local Go club contact information and from any neighbouring cities from where people may come. It may we worth producing your own local flyer giving contacts for your club and a few neighbouring ones. Here is a sample flyer which was produced for an event in Leicestershire. Always mention to people the BGA website, as printed on the leaflets, for a source of information about where to play and so on.
If you are not selling sets, but do have a local games shop that sells Go sets, make sure you have their details to pass to local people who want to buy. Otherwise recommend Pentangle via the BGA web site.