What system will you be running for these?
I suggest Mutants & Masterminds and the Mecha and Manga supplemental book.
I haven't completely decided yet. The problem is that there are three potential problems that I'm trying to avoid:
The first one is character creation. Most RPGs have fairly elaborate character creations and that takes time. As someone who is experienced with d20 systems (in all flavors), I can pop out a character pretty fast. However, for people who AREN'T experienced, it can take anywhere from 15 to 30 minutes assuming an experienced person can hold their hand through character creation. It'd take even longer if there isn't someone to help. In a convention setting, people are going to be coming and going all the time as panels begin and finish. As such, a simple character generation system is preferred. Granted, I could just make everyone use pre-made characters but I'd rather avoid that if at all possible.
The second issue is not everyone will know the system I'm using (no matter what system I use). d20 systems (D&D 3.5, D&D4e, Mutants and Masterminds, etc) are all relatively simple systems but they aren't "Here are all the rules in 5 minutes" simple. So with the forementioned time constraints, there really wouldn't be enough time to teach every person who comes by and I'd rather not turn that many people away. Even with people who are already familiar with the system I'd be using, I'd have constant rules disagreements with people over their interpretation of the rules and "Well, in MY group we play it THIS way" type arguments. So I'd have to pick a system that's even simpler because simple = less to argue about.
The last issue is a matter of cost/logistics: To play RPGs, you need books. You need them to make characters, you need them to double check rules and abilities, and many other things. Having a single books and then sharing it would work but it would slow everything down (and would see a LOT of wear and tear). Photo-copying important pages is an option but is 1) somewhat illegal and 2) Would require a LOT of paper (again, the cost thing. As such, I'm going to (primarily) stick to RPGs that are either free or are partially free.
My current plan is to run one of three systems:
Risus,
Pokethulu , or
Trigger Discipline . They're all flexible but simple systems. They don't have the variety or depth of other RPGs but they should work for the purposes of this convention.
I have experience with d20 (most flavors), Exalted, Cyberpunk 2020, Shadowrun (some), and a few other systems so if several people come up to me and want me to run one of those, I can (if they have the physical books with them, I only have physical copies of Exalted and D&D 3.5).