We've found that T-shirts are quite costly, but if enough people really want them we'll investigate. Stickers and keyrings are fun and not very expensive (Stickermule). We'd like to find a source of good quality mugs, like those which were given out at LinuxFest.
Basically it would be fun to have things like "I Love My Libre Tea", "My Other Computer Is Modular", "EOMA68: The Eco-way To Divide And Conquer", so we'd love to hear some suggestions that you'd like to fund. Let us know what you'd like! We have some ideas as well: some simple examples are included here.
Reach out to us on the mailing list.