Costa Rica packs an extraordinary range of terrain into a small country, and trekking is one of the best ways to experience it. The national parks are well maintained and most have marked trail systems, though venturing off-trail in dense rainforest is a serious undertaking — hire a guide if you plan to do anything beyond the main loops.
The marquee trek is the ascent of Mount Chirripó (3,820 m), the country's highest peak. It's a two-day effort with an overnight stay at the basic Crestones base camp. You'll need to book the hut in advance through SINAC, and spots go fast in dry season (December–April). The summit rewards with views of both the Pacific and Caribbean on a clear morning.
For something less strenuous, Corcovado National Park on the Osa Peninsula offers multi-day guided treks through some of the most biodiverse lowland rainforest on Earth. Monteverde and Arenal have excellent day-hike networks through cloud forest and past volcanic features. Rincón de la Vieja combines volcanic terrain with hot springs and waterfalls on well-marked loops.
Bring proper rain gear regardless of season — afternoon downpours are the norm everywhere except the driest months.