We recently returned from our 3rd visit to this area. Each time we have stayed longer and this time four days still wasn't enough for this amazing place.
Several things make this an incredible experience: first of all, it is absolutely beautiful. The boat ride San Carlos across Lake Nicaragua is pretty enough but once you enter the Rio Papaturo which leads you into the refuge, it becomes jaw-dropping for its scenery and abundant wildlife.
Once you get to the area of the Central Ecologico in the small village of Papaturo, there are only two places to stay and we by far prefer the Cabanas Caiman to the shared facilities of the Centro itself. The cabins are run by local guide extraordinaire Armando and his wife Aillen. To contact them you can send an email to firstname.lastname@example.org or call their cell phone numbers (506) 87043880 (Costa Rica) or (505) 86762958 (Nicaragua). Be patient because reception if very spotty down there. You will also have to speak Spanish. I can't recommend them enough.
Their two adjoining cabins are right on the river and very comfortable and clean (and even have electricity). They prepare meals in their outside kitchen and you get an incredible feel for the way the locals live.
The wildlife in the area is consistently the best we've seen in frequent trips to Central and South America and Armando's skills are unequaled. He's lived there his whole life and can identify every bird, insect, flower, tree, animal, snake, frog, monkey, etc. And he knows where to find them. He offers a variety of tours on his boat or on foot, day and night.
One of the highlights on this most recent trip was Armando finding a fer-de-lance deep inside a hole during a night tour to find sleeping birds. The next day, we came across a caiman eating a fer-de-lance in a very nearby spot. Armando got us amazing views (and resulting photos) but kept us safe the entire time.
I haven't even mentioned the great food prepared by Nidia and Aillen and the warm people in the area. There are a couple of local bars to grab a drink or bite to eat in walking distance too but you will feel very remote and isolated in this area.
You can get to the area by an inexpensive public boat that runs a few days a week or more quickly in a more expensive private boat available for hire in an San Carlos. We used Empresa San Miguel run by Miguel and his sons. Their number is (505) 88480505 or (505) 88286865 (again, you'll have to have someone who can speak Spanish on the call).
Once you get to Cabanas Caiman, you can get by even if you don't speak Spanish. Just bring great flashlights.
This area is amazing for birders which is why we keep going back.