Muisne is an island surrounded by the Muisne River on one side, and the Pacific Ocean on the other. The beach is beautiful, most often vacant, and lined with palm trees. It's off the tourist beaten trail and definitely worth a visit of a few days. To get there you can take a bus to El Relleno and take a motoeized canoe across Río Muisne to the island (20 cents). From there grab a mototaxi to the beach ($1, 2km) where you can find accomodation.
Muisne is an excellent place to visit the mangroves, and the organization FUNDECOL offers tours which help communities who have been affected by their destruction due to the industrial shrimping industry. A 4 hour tour goes for about $25 per person (two people, with prices decreasing from there). They also offer overnight trips to surrounding communities with cabañas at various prices (usually about $7 a night for accomodation.)