Expo 86, the Vancouver world's fair, already appears to be the big hit of this summer's travel season. Advance ticket sales have exceeded expectations, but the city claims it still has room for more visitors.

*Within 10 days of its opening last month, the fair welcomed its millionth visitor -- a family from Portland, Ore., with three-day passes. More than 15 million advance tickets have been sold, and officials now have raised their estimate of total visitors to 20 million before closing day Oct. 13 -- up from earlier predictions of 15 million.

Expo's early success is only one example, however, of what is shaping up to be a booming year for travel in North America -- the result of low gas prices, worries about terrorism abroad and the weakened dollar in Europe. For example:

*Orlando, home of Disney World and Epcot, is expecting "a really strong summer," says Paul Steiner, the city's tourism director, who advises visitors to have a hotel reservation before they show up. If they don't, they probably will find a room -- there are about 57,000 hotel and motel rooms in the area -- but it might take a lot of work.

*Ticketron, which takes campsite reservations for 10 national parks (beach and mountain), estimates a 10 to 12 percent increase in bookings this summer over last. One of the busiest parks (and not coincidently, one of the loveliest) is Yosemite in California, which is filling up regularly. But there is hope for late planners. Reservations can be made at Ticketron offices or at the 10 parks only eight weeks in advance. So August is still open.

*Amtrak is predicting a record summer. Its current bookings for the period from mid-June to the end of July are up 49 percent over the same period last year.

Last year Americans in Europe found themselves surrounded by their fellow countrymen. This year they already are bumping into them in Vancouver. So far, however, Expo seems to be handling the crowds well, but there have been lines, particularly for popular exhibits -- among them the U.S., Soviet and China pavilions -- and the monorail train that runs throughout the three-mile-long, 165-acre fair site on False Creek. Twenty-minute waits to board the Disneyland-like monorail are not uncommon.

In mid-May, the largest daily attendance was about 132,000 on a Saturday. When schools let out, the figure for a weekend day could top 200,000.

But officials are taking steps to reduce long waits where possible, says spokesman George Madden. For one thing, sightseers are being urged to use the seven free ferries that scurry back and forth on False Creek alongside the fairgrounds, docking at major exhibit areas.

At least two pavilions have begun issuing tickets with a specified time for admittance: the Great Hall of Ramses II, displaying treasures of the pharaoh from ancient Egypt, and Expo Centre, a 17-story geodesic dome with the largest Omnimax film screen in the world. Other pavilions are expected to adopt similar controlled-admission policies if the size of the midsummer crowds warrants it.

Some of the better sit-down restaurants at Expo, including the popular Swiss pavilion dining room, are taking dinner reservations. But getting something to eat has been no problem, says Madden, because more than 70 food establishments are scattered over the Expo site, including several McDonald's -- one of which sits on a barge floating in the creek.

Expo officials have been conducting exit interviews, asking departing visitors for comments. The biggest complaint to date, says Madden: "Not enough washroom signs."

July and August, not surprisingly, are expected to be the months of heaviest attendance, especially on weekends. Travelers who can wait until September will find Vancouver's climate still quite mild and fewer lines at Expo.

There are more than 80 pavilions on the grounds, including more than 50 from foreign nations as well as exhibits from most of Canada's provinces and territories and from three U.S. states -- California, Oregon and Washington. Seeing them all in a day would be a daunting goal, and probably an impossible one -- though the exhibits are open daily from 10 a.m. to 10 p.m.

Madden advises visitors who have the time to tour the fair in segments -- the European plaza one day, the Asian plaza another. For those in a hurry, the solution is to plan to see only those exhibits that interest you most. One way to cover a lot of territory is to take a rest break in the late afternoon at your hotel and return in the evening.

Only the exhibits close at 10 p.m. The rest of the fair -- the amusement rides, restaurants and concessions -- remain open until midnight for "Expo After Hours." The only catch is that you have to be inside the fair gates by 10 p.m.

If you are planning to go:

*Tickets: Discounts no longer are available for advance ticket purchases. Tickets can be bought at the fair gates. A one-day ticket is $15 (U.S.) for everyone 6 and older; there is no charge for children 5 and under. A three-day pass for adults 13 and older is $34.95; for children 6 to 12 and seniors 65 and older, the three-day pass is $17.50.

*Accommodations: Travelers with some flexibility in their arrival dates and who are willing to stay in hotels that are not luxury class should have no problem getting convenient lodging, according to Jon DeWest, who heads ResWest, British Columbia's official reservation service.

ResWest represents almost all of Vancouver's accommodations, says DeWest, including hotels, motels, campgrounds, bed-and-breakfast homes and four cruise ships docked in Vancouver for the duration of Expo. The booking fee is $5, which can be paid with a charge card. If all the commercial lodgings are taken, ResWest can put you in a private home at a rate of $35 a couple and $5 for each child. About 5,000 guest homes have been registered and inspected.

Vancouver hotel rooms are still readily available for June and September. The situation is tighter in July and August, but there still is space. According to DeWest, many hotels which had held back large blocks of rooms for potential conventions and other large groups have now released the rooms to ResWest.

Worried that travelers may think Vancouver is booked full, the local travel industry is taking out ads to assure Americans they will find a place to stay.

ResWest operates daily 7 a.m. to 11 p.m. (Pacific Daylight Time), (604) 662-3300. Because of the volume of calls, the line is often busy. The best time to phone is after 7 p.m. PDT (10 p.m. EDT).

Tours: A variety of tours that include Vancouver and Expo are being offered. They generally are available through travel agents.

An example is the "Northwest" package put together by Saga Holidays, which caters to travelers 60 and older. The 14-night motorcoach tour begins in San Francisco (three nights) and then heads north to the Napa Valley wine country; Redwood National Park on California's northern coast; Portland; Seattle; Vancouver and Expo (three nights); Canada's Banff National Park; and Calgary, the end of the tour.

Saga has frequent departures in September. The price is $1,359 per person (double occupancy), which includes round-trip air fare from Washington, 14 nights lodging, 14 breakfasts, 10 dinners and a number of sightseeing excursions. For information: (800) 343-0273.

Pacific Coast Cruise Lines is offering weekly seven-day cruises out of Seattle to Vancouver aboard its small, 79-passenger cruise ship Columbia, beginning June 28 and continuing until after the end of the fair. On each trip, the Columbia will dock two nights in Vancouver as a "floating hotel" near the Expo site.

Other ports of call are Port Townsend, Friday Harbor and Lake Washington in Washington and Victoria and Salt Spring Island in British Columbia. The cruise fares begin at $1,365 per person (double occupancy). For information: (800) 824-6170.

*ALL THAT JAZZ: Felix Grant, host of WRC radio's "Felix Grant's World of Jazz," is leading a 10-day tour July 10 to 20 to two major European jazz festivals, the Montreux International on Lake Geneva in Switzerland and the North Sea Jazz Festival in The Hague, the Netherlands.

The Montreux festival, in its 20th year, features 75 concerts by 1,000 musicians from around the world. At the North Sea, the concerts begin at 6 p.m. daily and continue until 4 a.m.

The cost of the tour is $1,690 per person (double occupancy), which includes round-trip air fare from Washington; accommodations in first-class hotels; breakfasts; festival tickets; sightseeing and other tourist activities. For information about this and other jazz tours in Europe: Vista Travel, 1735 Connecticut Ave. NW, Washington, D.C. 20009, 328-9555.

TRAVEL BOOKS: An unusual travel bookstore recently opened in the tiny Northern Virginia village of Hillsboro (population about 105) just south of Harpers Ferry on Virginia Rte. 9.

Called Travel Books, it features not the standard guides such as the Fodor and Frommer series, says co-owner Sheela Lampietti, but "the unusual guides, the out-of-the-way guides."

The selection includes books on adventure and budget travel and guides to places and countries off the well-trod tourist path. Additionally, the shop stocks novels by destination -- books set in Rome, for example, to read while you are visiting Rome.

There also are histories, art and architecture guides and a selection of travel literature -- accounts of exotic journeys old and new.

An out-of-the-way shop itself, Travel Books occupies the former tavern of Hillsboro, a village dating from the 18th century. It is open Saturday and Sunday from 11 a.m. to 6 p.m.

A mail-order catalogue is available by contacting Travel Books, P.O. Box 1259, Purcellville, Va. 22132, (703) 668-6952.

GREAT BARRIER REEF: Noted mountain climber and guide Arlene Blum, author of "Annapurna: A Woman's Place," will come down from the heights in October to lead a tour to the underwater world of Australia's Great Barrier Reef.

Limited to 18 participants, the 14-day trip will include lessons for beginners in snorkeling, scuba-diving, navigation and windsurfing. The only requirement is an ability to swim. Days will be spent diving, hiking the barrier islands, sailing or relaxing on the beach.

*Accommodations will be on 65-foot sailing yachts anchored each night in protected coves. A staff of eight guides and crew accompanies the tour.

The tour departs Oct. 18. The full land cost is $1,990 per person. Round-trip air fare from the West Coast is about $1,250. For information: Arlene Blum's Great Himalayan Treks, c/o InnerAsia, 2627 Lombard St., San Francisco, Calif. 94123, (800) 551-1769.