Weekly or monthly rentals are easy to arrange, both in San Juan del Sur [1] proper and in the hills surrounding the city. For the low budget set, most Spanish schools [2] in town have packages combining lessons and a homestay, which can be arranged (whether or not you are attending class) for around $10 per day including three meals. Most places will cut you a deal if you stay more than a few nights.
For something fancier see Vacation Rentals Nicaragua (www.vacationrentalsnicaragua.com [3]), Vacation Rentals by Owner (www.vrbo.com [4]), or Home Away (www.homeaway.com [5]), or stop by Aurora Beachfront Realty (tel. 505/2568-2498, U.S. tel. 323/908-6730). A fully furnished two-bedroom place goes for $800-1,200 per week and a big house is double that. Nearly all homes come with swimming pools and sweeping ocean views.
Links:
[1] http://www.moon.com/destinations/nicaragua/southwest-coast/san-juan-del-sur
[2] http://www.moon.com/destinations/nicaragua/essentials/tips-travelers/spanish-schools-nicaragua
[3] http://www.vacationrentalsnicaragua.com
[4] http://www.vrbo.com
[5] http://www.homeaway.com