Pack a light rain jacket, shoes that can get wet, and reef-safe sunscreen. In the Pacific dry season, December to April, you can skip a heavy shell. You still want the jacket. Valley nights usually sit around 18–26°C. Beaches commonly reach 28–32°C and do not cool off much after dark. The cold exception is Monteverde, and the summit of Chirripó.
Five quick-dry shirts, two shorts, one pair of long pants, two swimsuits, sandals, and shoes you can walk a wet trail in. Add water shoes for waterfall rock and a rocky entry. A hat matters by noon, because shade on the Pacific beaches is thin. A light fleece covers valley nights and a Monteverde morning. A rain jacket beats an umbrella on a boat and on a trail. Cotton stays wet once a shower starts at lunch. Leave the heavy coat and the jeans.
Reef-safe sunscreen is the one to bring. Some parks do not want the other kind in the water. Insect repellent matters on the Caribbean and in wet forest, and less on a dry Guanacaste beach at noon. The month you are matching is in the best time to visit.
Your passport, a paper copy, and a photo of the photo page on your phone. The original stays in the room safe. Carry the copy on a trail or a boat day. Bring the cards you will actually use, and any prescription you already take, in the original bottle. A pharmacy in town can help with a headache, a stomach, or a blister. It cannot replace a medicine you already depend on.
Add a small kit, not a suitcase of one: oral rehydration, an antihistamine, blister plasters, and something for motion if the ferry or the road to Monteverde bothers you. Private clinics in the valley and in the beach towns handle anything more. The public clinic is not the first door for a visitor. What not to leave in a parked car is in is Costa Rica safe?.
A dry bag, a hat with a cord, a light long sleeve, and the rain jacket at the top of the bag, not under the clothes. Boats leave in the morning from Tortuguero, from Uvita for whales, and from the bays in the north. The sky can be clear at the dock and wet an hour later. Bring the cash for the ride before you leave town. There is no ATM on the boat.
On a wet trail, closed shoes beat sandals. Monteverde is mud in every month. A waterfall stair, such as the one in La Fortuna, is slick on the way down and the same stair on the way up. An extra pair of socks and a plastic bag for the wet shoes are worth more than a second outfit. Clothes that dry overnight matter on the Caribbean too. Rain there is not limited to the green season. Puerto Viejo is not a dry-season exception.
December to April on the Pacific is sun, hotter beaches, and less rain. You can skip a heavy shell. You cannot skip sunscreen, a hat, and water. Around Tamarindo the hills are brown by March, and there is almost no shade at noon. You still want the light layer. Alajuela and San José cool off at night, and Monteverde stays misty in a dry week.
The first swim is the morning. Afternoon wind puts a chop on the bays, and the sun is what ends a long sit. Pack for that, not for a cool evening on the sand.
The heavy coat, the jeans, the extra shoes, and anything you would hate to lose from a parked car. A mask and a surfboard are easy to rent in a beach town. A mat is easy to borrow where yoga is the morning, in Nosara and Santa Teresa. A hair dryer and a second pair of nice shoes do not earn the space.
Park gates, buses, and small sodas still prefer cash, in small colón bills. Take it out before a Sunday or a boat day. Dollars are often accepted. The change comes back in colones.
