Opened in 1971, Grendel’s is one of the few vestiges of Harvard Square’s bohemian heyday. The underground space is indeed denlike, with a cozy fireplace and sometimes-spacey service. A young crowd flocks here for the half-price, vegetarian-friendly food menu (daily 5–7:30pm and Mon–Thurs 9–11:30pm)—but note that it’s not available on the pleasant patio, which faces a small park.