Lamanai is a great trip from the island. Tanisha Tours and Searious, among others, offer this all day trip. There are several snorkeling trips offered as well, including Bacalar Chico (around the north of the island) and a day trip to Caye Caulker. Tikal is fly only, and very expensive. If you only have a week, might want to save it for next trip, and base for part of the time in San Ignacio.