There will be tourism, and prestige luxury merchandise (Zubrin submits that the settlers might export artificial Martian diamonds tinted with a regolith hue). Perhaps they could sell the broadcast rights to low-gravity spectator sports like rover racing or something "comparable to the exhilarating Quidditch games" in Harry Potter; on Mars, Zubrin points out, "basketball players will be able to jump three times as high."
