You won't have to show your return ticket. Technically you're supposed to have one but I've never seen a tourist asked for one. Just hand the Immigration official your passport and visitor card which you'll get on the plane or at the desk when you're leaving the U.S., and hold your ticket jacket. If asked how long you're staying, say about a month. Before your leave the desk, look to be sure that the official gives you the full 30 days. (Note that the way dates are written in Belize is day-month-year, not the U.S. format. If not, ask politely for 30 days. Really it's not a big deal. You have the right to be in the country for 30 days and the right to renew your entry card.