If you want something to do besides loll around on the beach all day, visit during festival time when things get lively. In Maunabo [1], the big draw is Festival Jueyero, a celebration that fetes the land crab, held in late September in the town plaza on Calle Santiago Iglesia. All sorts of crab dishes are on the menu, as are a parade, crab races, and more.
Maunabo is also home to Fiestas del Pueblo, a town celebration held in late June in the plaza featuring music, dance, rides, games, and food kiosks.
Yabucoa [1] has a Patron Saint Festival, held in late September through early October in Parque Felix Millan, to commemorate Saint Angeles Custodios.
Links:
[1] http://www.moon.com/destinations/puerto-rico/east-coast/yabucoa-maunabo-and-patillas