What are the most worthwhile things to do in Ambergris Caye on a first visit?

Start with the trip the island exists for. Hol Chan Marine Reserve, about 4 miles south of San Pedro, was established in July 1987 as Belize's first marine protected area and now covers 102,400 acres in eight zones. Boats anchor at the Hol Chan Cut, a natural break in the reef about 75 feet wide and 30 feet deep where grouper, snapper and sea turtles move between the lagoon and the open sea, then continue to Shark Ray Alley — added to the reserve in 1999 — where nurse sharks and southern stingrays circle in about 6 feet of water while snorkelers watch from the surface. The reef itself is part of the Belize Barrier Reef Reserve System, inscribed by UNESCO in 1996 as a seven-site property and the largest barrier reef in the northern hemisphere.

The second signature trip goes much farther out. The Great Blue Hole, a collapsed limestone cavern 318 meters (1,043 feet) across and 124 meters (407 feet) deep, sits in Lighthouse Reef about 43 miles (70 km) from San Pedro. Divers descend to about 130 feet to see stalactites in the flooded cave; it is an advanced dive, and non-divers get the better view anyway on a small-plane flight, where the dark circle reads perfectly from the air. Closer to home, Mexico Rocks, off the island's north side and folded into Hol Chan's expanded zone system in the reserve's later expansion, is a sheltered patch-reef snorkel of coral heads and rays in calm, shallow water — the gentler alternative when the outer reef is windy.

On land, Ambergris Caye runs on golf carts: rent one and cross the Boca del Rio bridge into north Ambergris Caye, where the road runs past beach bars to Secret Beach on the leeward west shore, a shallow, gin-clear swimming flat with in-water picnic tables. Anglers should book a flats guide for the island's third signature pursuit — fly-fishing the turtle-grass flats for bonefish and permit, with tarpon in season, the combination locals call a grand slam when all three are caught in a day. Finally, ride the water taxi about 30 minutes south to Caye Caulker, walk its sand streets under the "Go Slow" signs, and swim at the Split, the channel a 1961 hurricane cut through the island, before the boat back.

At Shark Ray Alley the boat engine cuts and the water, barely deeper than a person is tall, fills with shapes: slow nurse sharks the color of wet sand, rays lifting off the bottom in puffs of white, and a rolling boil of jacks where the guide has thrown chum.

The whole of Ambergris Caye was once attached to Mexico's Yucatán Peninsula — the channel separating the island at its north end, at Bacalar Chico, is credited to Maya traders who cut it by hand roughly 1,500 years ago to shorten their coastal canoe route. Bacalar Chico National Park and Marine Reserve, at the island's far north, is one of the seven UNESCO-listed sites, and it is the one almost nobody on the island visits.

• Day one on the reef — Snorkel Hol Chan and Shark Ray Alley on a morning trip out of San Pedro
• One big excursion — Choose the Blue Hole by boat if you are an advanced diver, or by air if you are not, rather than both
• One slow land day — Golf-cart north to Secret Beach, then take the water taxi to Caye Caulker for sunset at the Split

The Blue Hole crossing is roughly 2.5 hours each way in open sea and is cancelled in strong wind, so it should never sit on your last day. Hol Chan charges a park fee collected through tour operators, snorkeling there is guide-only, and touching or standing on coral inside the reserve is prohibited. Golf carts cannot be taken just anywhere off the main roads after rain, when north-island tracks flood.

Book Hol Chan for your first morning, hold the Blue Hole trip for the calmest forecast day, and keep one unscheduled day for the cart-and-Caulker combination.