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

Pearl Harbor National Memorial charges no entrance fee, and the visitor center and its grounds are free to enter. The memorial program over the sunken battleship is the exception, and not because of money: a non-refundable management fee of $1.00 per ticket applies through the federal recreation booking system, and tickets are released in two batches — eight weeks ahead, and again 24 hours ahead. The last boat of the day leaves at 15:30. Those two release windows are the whole game; there is no standby line that reliably substitutes for them.

Diamond Head is the other fixture, and it is a state site rather than a federal one. Non-resident admission is $5 for entry, with a separate $10 charge for a non-commercial vehicle; commercial vehicles are tiered by capacity at $25 for one to seven passengers, $50 for eight to 25, and $90 for 26 or more. The summit trail runs 0.8 miles from trailhead to summit with 560 feet of gain, and the state allows 1.5 to 2 hours for the round trip. The monument covers over 475 acres, taking in the crater floor and the outer slopes.

The crater itself is the oldest thing on any Honolulu itinerary. It formed roughly 300,000 years ago in a single explosive eruption, and the ash and fine particles that settled afterward cemented into tuff — the rock the trail is cut into.

Downtown, the Chinatown Historic District was added to the National Register of Historic Places in 1973. It is bounded roughly by Nu'uanu Stream, Beretania Street, Nuuanu Avenue and Honolulu Harbor, and covers about 35 acres — walkable in an afternoon and a complete change of register from the beach.

Offshore, the water beyond the harbor lies inside a federal marine sanctuary established for humpback whales, which covers the relatively shallow water — less than about 600 feet deep — around the main Hawaiian islands. That is why whale-watching departures from Oahu are a seasonal activity with a published basis rather than a marketing claim.

The Diamond Head trail climbs the inside wall of the crater, so for most of the way up you are looking at rock rather than ocean; the view only opens at the rim. That rock is tuff — compacted ash and fine particles thrown out by the single eruption that built the cone.

The 24-hour ticket release for the USS Arizona program is the quietly decisive detail. Travelers who miss the eight-week window often assume they are out of luck for the entire trip, when in fact a second tranche of tickets appears a day ahead — which means a flexible visitor can still get aboard, but only by checking on the right morning rather than hoping at the door.

• Eight weeks out — Put the memorial program reservation in the calendar before flights; everything else on Oahu can be arranged later.
• Dawn at the crater — A 0.8-mile climb with 560 feet of gain is a different proposition at 07:00 than at noon on an unshaded slope.
• Afternoon downtown — Thirty-five acres of registered historic district is the right size for the hours after a morning site visit.

The memorial program is weather- and sea-state dependent, and the last departure at 15:30 leaves no margin if your morning runs late. Non-resident admission at Diamond Head bundles entry with a parking reservation, so the two signature sites both have to be arranged rather than visited on impulse. Whale-watching is seasonal and its timing is set by the animals, not the operator. And the $1.00 ticket fee is non-refundable, which matters if your plans are provisional.

The Honolulu when-to-visit question in this group lists both sites' daily hours and their annual closure dates, which determine whether your travel week works at all.