This secluded and exclusive two-block-long East Side enclave has an aura of imperturbable calm. Residents of its elegant town houses have included the Rockefellers; Alfred Lunt and Lynn Fontanne; Ethel Barrymore; Irving Berlin; and, of course, Auntie Mame, a character in the well-known Patrick Dennis play (and later movie) of the same name. Steps at East 51st Street lead to an esplanade along the East River. Subway: 6 to 51st St./Lexington Ave.; E, V to Lexington-3rd Aves./53rd St.