I think you (and frob) already hit the nail on the head; you're going to have to pick a small part of the game that's sufficiently interesting to get people interested and focus on that. They're simply not going to get to experience everything the game has to offer, so your goal should be to pick something that's interesting enough to draw them in to further research, and to make that further research as easy as possible. I would:
- Make a nice landing page on your website that lists the core features of the game, with short to-the-point (as much as possible for any given feature) videos of each and concise explanations.
- Everyone who stops by at the convention should at minimum get a business card with the url to this landing page, preferably with a QR code for convenience. The rest of the card design needs to be memorable enough to associate with whatever you show off.
- You might also give them a printed leaflet showing off some of the major features that may have been missed at the convention.
- Try to build an email list with everyone that stops by. Figure out how to make sign-up as quick and painless as possible, and have very quick and clear privacy statement ready so they know they won't be getting spammed; obviously, also don't spam them.
How many players do you intend to have, and how many people demonstrating? You need to find the most exciting/gripping/interesting experience you can offer, which as you've said unfortunately may entail a bit of a steep learning curve and might swamp the players with too many options. You may be able to alleviate the learning curve a little by personally talking each player through any difficult parts of the experience.