Just came back from Guanacaste as well, though I stayed in a condo and not the Westin. However, I absolutely recommend venturing out of the hotel for some great seafood at the nearby restaurants. Especially great was Pollo Tropical, an unassuming location nearby the Westin with a super-friendly owner and a family-run kitchen. Despite its name, the standout dishes were fish, especially the whole red snapper. I only regret not discovering this place until our last day, so we were only able to go back once more for dinner before we departed.