House Without a Key   House Without a Key, a seaside open-air dining room and courtyard, is this exclusive hotel's version of a coffee shop, serving a generous daily breakfast buffet and lunch and dinner of the classic American sort (burgers, salads, sandwiches at lunch; grilled meats and fish and pasta at dinner and Tuesday night open-air grilling). The draw is the setting and the nightly live music and hula; it's the premier spot to bring out-of-town guests for sunset, drinks and a gracious Hawaii experience.  Halekulani hotel, 2199 Kalia Road. 923-2311. 7-10:30 a.m, 11 a.m.-9 p.m. Mondays-Saturdays; 7 a.m.-9 p.m. daily. Valet and self-parking. No reservations. AX, DC, JCB, MC, VS. No checks. $$$.  Web site: www.halekulani.com