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

The park's published trail list is short enough to plan in full. The Costa Rica Tourism Board gives Las Coladas as 1.7 kilometers one way, allowing about an hour including the 20 minutes at the lava viewpoint; El Ceibo as 2.3 kilometers in roughly an hour and a half; Los Miradores on the peninsula as 1.2 kilometers in about an hour; and Heliconias as 1 kilometer in about 25 minutes. The main lookout trail is listed at 1.8 kilometers. That is the whole network — a full day here is four short walks, not one long one, and the park's 8:00 to 16:00 window accommodates all of them.

Las Coladas earns its priority because of what it crosses. The Global Volcanism Program's record for Arenal counts more than 40 discrete lava flows laid down by 1983 alone, from an eruption that opened on the night of 29 July 1968 and buried a wide area of Guanacaste Province in ash. Walking onto that surface is the closest thing the park offers to standing inside the event.

Los Miradores, on the peninsula, is the trail to choose for the other geography: the park spans altitudes from 500 to 1,633 meters, and the lower, lake-facing ground gives the long view west over Lake Arenal rather than up at a cone that may be in cloud.

On the lava trail the footing changes from forest soil to angular black blocks within a few paces, and the blocks ring slightly underfoot where they have not settled. Above the flows the forest edge is abrupt — a clean line between what burned in 1968 and what did not.

Arenal is not a World Heritage property, and that is worth knowing before you plan a trip around it. Costa Rica has four properties on the World Heritage List — Cocos Island National Park, Área de Conservación Guanacaste, the Talamanca Range-La Amistad Reserves with La Amistad National Park, and the Precolumbian Chiefdom Settlements with Stone Spheres of the Diquís — and Arenal is none of them. The volcano's status is national, not international, which also means its rules are set locally: card-only payment at the gate, identity document required, no pets, and no leaving the marked trails.

• First walk — Las Coladas, for the lava. Twenty minutes at the viewpoint is what the park allots; take all of it.
• Second walk — Heliconias, 1 kilometer and about 25 minutes, as the short filler between longer trails.
• Last walk — Los Miradores on the peninsula, 1.2 kilometers, for the lake rather than the summit.

Everything inside the park has to happen between 8:00 and 16:00, which rules out both dawn and dusk. The summit has not produced visible eruptive activity since the sporadic Strombolian explosions of December 2010, so night-viewing excursions sold on the idea of glowing lava are selling a memory. Costa Rica charges a departure tax of US$29 for leaving by air, normally included in the ticket price, and British passport holders need an expiry date at least one day after the day they plan to leave the country.

Pair this with the when-to-visit answer in this group for the dry-season timing that decides whether the cone is visible at all, and with the food answer for La Fortuna.