Fliers Don’t Need to Be Frequent to Add Miles : Airlines: Travelers can now earn mileage credits by staying in certain hotels, booking rental cars, dining out and using credit cards.

In their eagerness to attract passengers, the nation’s airlines have created so many frequent-flier gimmicks that you can earn enough mileage credits for a free round-trip ticket without ever leaving the ground.

Could the airline executives who dreamed up the popular mileage programs a decade ago have envisioned this ironic twist to their brainchild? Hardly.

The latest mileage-building scheme is being initiated by the Marriott hotel chain--along with five U.S. airlines and British Airways. The offer, called Marriott Miles, awards participants 500 frequent-flier miles every time they stay at any of Marriott’s 250 hotels worldwide. But, unlike most other similar-sounding hotel mileage programs, you don’t have to fly to your destination to qualify for the miles. Instead, you can drive to a Marriott or hike, paddle a canoe or show up on horseback.

Marriott Miles is just one of the many ways alert travelers can accrue frequent-flier miles--that is, in addition to buying a ticket and flying from one airport to the next. Depending on the airline, you can sometimes add to your mileage account when you buy eyeglasses, make long-distance phone calls, contribute to a charity, charge your purchases on an airline-affiliated credit card, rent a car, book into certain hotels or even dine out in a restaurant.

Among the most unusual promo tions, Northwest Airlines is promising 500 mileage points in its WorldPerks frequent-flier program, through June, to anyone who donates $50 to Big Brothers/Big Sisters, a national service organization.

It should be no surprise that mileage credits mount fastest when travelers can take advantage of as many offers as possible. Some offers are long-standing and others of limited duration. Most airline frequent-flier programs mail out notices to members detailing new mileage promotions, although sometimes the information appears first in newspaper ads.

Travelers also can keep tabs on the promotions by subscribing to one of several frequent-flier publications, among them “Best Fares” (800-880-1234), “Frequent Flyer” (800-323-3537, Ext. 106) and “The Flyer’s Edge” (800-248-1826).

Among the many, and growing, ways to earn mileage credits:

* Marriott Miles:

Marriott introduced its new promotion last month. It is an alternative to Marriott’s decade-old Honored Guest Awards program, in which frequent guests can earn free vacations at the chain’s 250 properties. To participate in Marriott Miles and Honored Guest, guests must sign up for each of the programs separately, and every time they check into a hotel they can specify whether they want to earn mileage credits for an airline ticket or points for a Marriott vacation.

Under the Marriott Miles promotion, guests earn 500 frequent-flier miles on American, Northwest, USAir and British Airways for each stay. A stay can be as short as one day, but multi-day stays still qualify only for a single 500-mile credit. After five stays, they earn a bonus of 2,500 miles (in effect, double miles) for a total of 5,000 miles.

It takes 20 stays at a Marriott to earn the 20,000 miles needed for a domestic round-trip ticket on American, Northwest and USAir, although a guest can earn a round-trip ticket on Continental Airlines with just 10 stays. A round-trip ticket to Europe this summer on British Airways is 60,000 miles (60 stays).

Participation in Marriott Miles is free. Enrollment forms are available at Marriott hotel front desks, or you can sign up by calling (800) 367-6453.

* Charge cards:

Most major U.S. airlines offer what are called “affinity cards.” (Delta is an exception.) They can be either a Visa or MasterCard, and every dollar you charge (and pay for) on them earns a mileage credit toward a free ticket.

American Express has its own program called Membership Miles. You earn a mile for every $1 you charge (and pay for) on the American Express card. But you can apply the mileage to any one of four airlines--Continental, Delta, Northwest and Southwest. Before you can use any miles, however, you must accrue a total of 5,000 mileage credits--which is the equivalent of charging $5,000 on your American Express card. There is an annual fee of $25 to participate in Membership Miles, but it is waived during the first year of enrollment. American Express also charges a fee of $55 a year for its green card, the least expensive of its cards. To enroll in Membership Miles, call (800) 274-6453.

Diners Club has a similar program, although it permits you to earn mileage credits at the rate of one mile per $1 charged (and paid for) on 12 major U.S. airlines--or essentially all of them. The annual fee for the card is $80. For an application form, call (800) 234-6377.

* Hotel stays:

In addition to Marriott, a number of other U.S. hotel chains offer frequent-flier credits when you check in. Among them are Radisson, Hilton, Sheraton, Inter-Continental, Wyndham, Stouffer, Hyatt and Westin. Each hotel is associated with one or more airlines, called “partners.”

To qualify for mileage credits, generally you have to fly a partner airline and check into a partner hotel. The customary award is 500 mileage credits per stay, but at some hotels you can earn 1,000 miles if you fly on Delta and Continental. In slack periods, airline and hotel partners sometimes increase the amount of the awards to attract more business.

* Car rentals:

In much the same way, air travelers can earn mileage credits when they rent from a car-rental firm that is a partner of one or more airlines. Most major car-rental firms participate in frequent-flier programs, and the usual award is 500 mileage credits per rental--although some rental firms offer 1,000 miles to Delta passengers and 1,000 to 1,500 to Continental passengers. National Car Rental’s ongoing promotion with American Express and Continental this year is unusual in that participants are not required to fly Continental to qualify.

* Phone service:

American and Northwest have joined with MCI, a long-distance phone company, to offer frequent-flier mileage credits in yet another way. MCI customers earn five mileage credits for every dollar they spend on long-distance calls from a private residence. For information, contact MCI-American, (800) 999-1909, or MCI-Northwest, (800) 999-0280.