All good things do have to end. "The Fantasticks," the world's longest-running musical, will close June 8 after more than 26 years and 10,864 performances at off-Broadway's tiny Sullivan Street Playhouse. The show, with memorable songs like "Try to Remember" and "Soon It's Gonna Rain," is a spoof on "Romeo and Juliet," in which two fathers invent a feud in order to bring their children together.

"The Fantasticks" was written by Harvey Schmidt and Tom Jones while they were in college. Such diverse stars as Liza Minnelli, John Carradine, Tom Poston and Anna Maria Alberghetti have performed somewhere in the staggering 8,913 productions of the show, including amateur productions, in the United States that had been staged as of Jan. 1, 1986. There have been an additional 472 productions in 67 foreign countries. A television production in 1964 starred Ricardo Montalban, Bert Lahr, Stanley Holloway, Susan Watson and John Davidson.

Most people will correctly point out that it is not a wise investment, putting money in a Broadway or off-Broadway show. They're often only useful for investors looking for a tax write-off. When a show hits, however, all the advice is out the window. Since "The Fantasticks" opened, it has paid its backers an 8,242 percent return on their investment. On Guard With the Royals

In another sign of the times, more than 400 police and military security specialists are guarding Prince Charles and Princess Diana during their one-week stay in western Canada, which began yesterday. They are in Victoria, British Columbia, to open the $1.5 billion Expo '86 World's Fair. Officials are expecting 15 million to visit the fair during its six-month run, including many Americans who would otherwise have gone to Europe but for the recent terrorist attacks.

To prevent any attacks at the Expo, the security forces will sweep the routes the royal couple will take, examine the sewers and have hidden marksmen on rooftops. The Canadians are determined to prevent the kinds of attacks that are spreading throughout Europe. End Notes

Some 10 years before Live Aid concert organizer Bob Geldof became involved in the hunger issue, singer Harry Chapin was there. With William Ayres, he founded World Hunger Year and lobbied for the establishment of the Presidential Commission on Hunger, which began during President Carter's administration. He also donated more than half the profits from his concerts (reportedly more than $5 million) for hunger relief before he died in an automobile accident in 1981. His widow Sandy and his brother Jim and other friends and associates are to testify today before the House Banking Committee and the subcommittee on consumer affairs and coinage on a proposal introduced by Rep. Byron Dorgan to have a congressional gold medal awarded in his memory . . .

A lawyer for the woman who says she was the late Rock Hudson's adopted sister announced yesterday he is filing a suit against the late actor's estate contending his client was wrongfully left out of the will. The woman, Alice Waier, 32, of Grants Pass, Ore., has hired lawyers Melvin Belli and Paul Monzione to challenge the will . . .

The singer's singer Tony Bennett was not there to sing at the annual Wolf Trap and the Arts luncheon yesterday that honored Catherine Shouse, who will be 90 in June. Surprisingly, Bennett, who begins a three-night stand tonight at the Kennedy Center, was there to speak and show slides of his paintings, another talent for which he is becoming well known. Among the 300 guests were Midge Baldrige, Susan Baker, Gail Berendzen, Jeannie Baliles, Nuala Pell and Frank Hodsoll, chairman of the National Endowment for the Arts . . .

Washington State was the first to give women the right to vote, and in 1916, four years before the federal government granted women suffrage, Josephine Garber voted for the first time. The 90-year-old woman, who has voted in every election since, is in town this week campaigning for her grandson Leon Billings, the former Hill aide and former executive director of the Democratic Senatorial Campaign Committee who is running for Congress from Maryland's 8th District.