HONOLULU (AP) _ Prince Charles and Princess Diana remained secluded Friday during the first stop of a five-day visit to the United States, relaxing at a luxury hotel near Diamond Head before flying to Washington, D.C., for a weekend of festivities.

The royal couple had no public appearances scheduled during their 18-hour stop in Hawaii, where they arrived just before midnight Thursday after a 12- hour flight from Canberra, Australia.

Prince Charles and Princess Diana were free to do some sightseeing or ''to take it easy like other tourists,'' said John Houlton, a British vice consul for information from Los Angeles.

''It's up to them what they would like to do while they are here,'' he said.

The prince and princess of Wales were scheduled to leave at 6 p.m. HST Friday (11 p.m. EST) for a three-day visit to Washington, which includes a gala with President and Mrs. Reagan as hosts. On Tuesday, they fly to Palm Beach, Fla.

It is Princess Diana's first visit to the United States. Prince Charles last visited Hawaii in 1974 when he was in the Royal Navy.

The couple stayed at the beachfront Kahala Hilton Hotel, about two miles east of Diamond Head and well away from the bustling and crowded Waikiki Beach resort area.

Soon after a decoy motorcade left the hotel, the royal couple slipped out of the back entrance and were taken by limousine to a private beachfront residence a few blocks away where they ate a lunch provided by the hotel.

Reporters and photographers kept watch from across the street. Neighbors said they did not know who lived at the residence.

State security officers asked a work crew using a hydraulic cherry picker to trim tree limbs away from electric lines at the home next door to suspend their work, the workers said.

Security men patroled the grounds while others kept watch offshore in rubber dinghies.

Other hotel guests seemed to be ignoring the presence of the royal couple, although a few area residents admitted they had come to the hotel in the hope of catching a glimpse of the pair. About two dozen members of the news media, most of them British, waited outside the hotel.

Several photographers aimed high-powered lenses at the upper-floor windows where they believed the couple might be staying.

Prince Charles and Princess Diana had been expected to go directly to their limousine for the 20-mile trip to the hotel after their late-night arrival in Hawaii. Instead, they spent a few miniutes shaking hands and chatting with some of the nearly 300 people on hand to greet them at Hickam Air Force Base.

Princess Diana was given a five-strand pikake lei and Prince Charles recieved a three-strand cigar flower lei.

''Oh, how sweet they smell,'' the princess said.