House Without a Key   For many O'ahu kama'aina, there IS no other place to take visitors for a sunset drink and an evening of live music and hula than the lanai at House Without a Key — it's a trip back in time, to an era when Mainlanders would spend months at the nearly 100-year-old Halekulani hotel in the curve of storied Gray's Beach. But House Without a Key is also the hotel's casual breakfast and lunch dining room, conveniently located near the pool and lobby. Relaxed dinners are served here, too, when you don't feel like dressing up for a meal at Orchids or La Mer. But the ultra-gracious service is the same.  Halekulani, 2199 Kalia Road. 923-2311. halekulani.com; 7 a.m. 9 p.m. daily. Valet and self-parking. No reservations. AX, DC, JCB, MC, VS. Checks accepted. $$$.  Web site: www.halekulani.com