The activities in and around Kuri and Las Marias are endless.
To start with Kuri, the miles of endless beach makes for good swimming and long walks with great sunsets. Speaking and interacting with the locals is also an activity that will pass alot of time. The locals are always keen for a chat no matter what standard your spanish is at. The nearby mangroves are great to explore by boat with a local guide, especially at night when you can go looking for crocodiles by torch light, something not for the faint hearted. I better not forget for the less active of us, relax, take it easy, read a good book, the days will pass easier enough, trust me.
Las Marias, exploring the jungle is the first must do. The local guides, which can be organised know miles and miles of trails through virgin tropical forest also they are quiet knowledgeable of the forest. Second not far behind the jungle. Is visiting the petroglphs, stone carvings that where made by an unknown culture and for an unknown reason. A day boat trip up the river will take you to the first and second main set of carvings, during the trip you will experience the forest from a different point of view. The boats are a great way to observe birds and animals along the rivers edge. Throw in cultural interaction and relaxation and you have more than enough to fill in a few days or much longer.