The Hotel Cabo Blanco tennis courts (tel. 315/355-5182) are customarily open for public rental for about $4 per hour. Call ahead to check. Lessons may also be available. The Hotel Portico del Mar (tel. 315/355-6495) also has tennis courts for outside visitors ($3/hr).
The plumy, breezy 18-hole Isla Navidad Golf Course is available to the public for a fee upwards of around $100 per person. Call the Hotel Grand Bay (tel. 315/331-0500, www.islanavidad.com [1]) and ask for the Club de Golf for information. Get there by regular launch from the Barra dock on the lagoon (to the Casa Club landing, about $5 round-trip). By car, turn right from Highway 200 at the Ejido La Culebra (or Isla Navidad) sign as the highway cuts through the hills at Km 51 a few miles south of Barra. Follow the road about three miles (4.8 km) to a bridge, where the road curves right, paralleling the beach. After about two more miles (3.2 km), you pass through the golf course gate. After winding through the golf course another mile (1.6 km), turn right at the traffic circle at the north edge of the golf course. Continue another mile (1.6 km), between the golf course and the adjacent hillside, to the big golf clubhouse on the right.
Links:
[1] http://www.islanavidad.com