There's no bad time to visit the Caribbean Coast. During the rainy season, you  still get beautiful, sunny mornings, but be ready for some afternoon showers.
You'll find a relaxed, laid back atmosphere. Cahuita National Park is one of, if not the nicest of all the National Parks in Costa Rica. The flat terrain makes for a easy hike for all ages. The trails follow the beach, and there are a countless number of beautiful tall palm and shade trees along the way. You can spot howler monkeys, sloths, white-face monkeys and all kinds of crabs and interesting insects. 

Playa Negra

The warm waters are excellent for bathing and snorkeling. Next to Cahuita is Playa Negra (Black Beach) named because of the dark black sand beach distinct to the area. The waves make for great surfing, body surfing or just plain old frolicking in the waves. A little further South of Cahuita is Puerto Viejo, where you'll find great surfing waves at Playa Brava. If your more into adventure, try one of the many tours offered in the area. There are Canopy Tours, know as zip line, Fishing, Snorkeling, Jungle Tours, Tortuguero Trip, Bird Watching, Indian Reserve Excursions and more.