Light weight walking shoes and/or hiking boots will get a lot of use, but river sandals or water shoes and running sneakers will be useful for stream crossings and for light walking days. Take synthetic or wool socks.
Upper Body Clothing
Carry at least two sets of lightweight, fast-drying synthetic tops and T-shirts (light colors for minimal heat absorption) because high humidity reigns even in Costa Rica's dry region. Also take along a raincoat and a warm sweater and/or light jacket for colder, windier elevations. Pack clothing for a couple of days in your carry-on bags to allow for late-arriving checked items.
Lower Body Clothing
Light synthetic pants and hiking shorts (two each) will be practical. Consider leaving your jeans at home because they dry far too slowly. A swimsuit and towel are handy to use at the hot springs and beach.
Head and Hands
Include a sun-protection hat in your gear as well as a bandanna and two pairs of sunglasses. A wool cap and gloves will keep you cozy at elevation, and sunscreen (30 spf or higher) is imperative.
To enjoy the wealth of scenery, forests and wildlife without stress, don't forget insect repellent, binoculars, camera, passport, pesos, credit card, toiletries, flashlight or headlamp, water bottle and cup, first-aid kit, small backpack for day trips, Spanish phrase book, and field guide and animal identification books.
Lodges are available outside the park, but overnight stays inside the park consists of camping at ranger stations or dorm facilities at biological field stations. Pack camping gear or sleeping sheets as needed.