You can do the 16-kilometer hike to the Chirripó summit in Chirripó National Park  in a day, but it normally takes two days (three days round-trip). Call the National Parks Service three days in advance to register and pay your deposit; if you arrive without reservations, pay your fee at the ranger station at San Gerardo. There are distance markers every two kilometers. Pack out all your trash and bury human waste.
Today is 14 kilometers, mostly steeply uphill. Less-fit hikers should begin not long after dawn, as it can take 12 hours or even longer in bad conditions (more fit hikers should be able to hike this section in 6–7 hours). You can hire local porters to carry your packs to base camp.
From the soccer field in San Gerardo, walk uphill about 600 meters to the Y-fork; turn right, cross the bridge, and follow the rocky track one kilometer uphill. The trailhead is well signed on the right, 100 meters above Albergue Urán (you can drive up to this point with a 4WD vehicle; several homesteads advertise parking for a small fee). There’s a stream 0.5 kilometer beyond Refugio Llano Bonito, beyond which you begin a grueling uphill stretch called La Cuesta del Agua; allow at least two hours for this section. The climb crests at Monte Sin Fé (Faithless Mountain). You’ll see a rudimentary wooden shelter at the halfway point, beyond which you pass into dwarf cloud forest adorned with old man’s beard.
About six kilometers below the summit is a cave large enough to sleep five or six people if rains dictate. From here a two-kilometer final climb — La Cuesta de los Arrepentidos (Repentants’ Hill) — takes you to Centro Ambientalista El Páramo lodge, beside the Río Talari beneath an intriguing rock formation called Los Crestones. It has heating, but be prepared for a cold night anyway.
Today, get up and onto the trail by dawn to make the summit before the fog rolls in. It’s about a 90-minute hike from the hut via the Valle de los Conejos (Rabbits’ Valley). On clear days the view is awesome! With luck, you’ll be able to see both the Pacific and the Caribbean.
You can head back to San Gerardo the same day, or contemplate a round-trip hike to Cerro Ventisqueros, the second-highest mountain in Costa Rica (by permit only); the trail begins below the Valle de los Conejos. Another trail through the Valle de las Morenas, on the northern side of Chirripó, is off-limits without a permit, as is the Camino de los Indios, a trail that passes over Cerro Urán and the far northern Talamancas.