What can you say to a man whose favorite fantasy is that Mao Tse-tung visits the United States and "I am asked by the State Department to take him eating for a week in New York . . ."

You can say, Calvin, let's eat, which is what the Kitchen Bazaar is saying to author Calvin Trillin, whose third book, appropriately titled Third Helpings, has just been published (Ticknor & Fields, $12.95).

To promote his visit Thursday, the Kitchen Bazaar has sponsored a contest among its customers asking for everyone's favorite Washington food. Winning suggestions, i.e., all but the most improbable, will be announced at the Trillin demonstration and book signing.

Judging by his fantasies, Calvin Trillin is a man who would prefer a Washington munch to a Washington monument. The enthusiasm that Kitchen Bazaar customers showed in standing up for their favorite foods indicates he's not alone.

Let others tiptoe through the tulips on the Mall, ponder the wings of man in the National Air and Space Museum or gaze at the pictures in the National Portrait Gallery. The next time an out-of-town visitor arrives, take them on a food tour of Washington--marble cake instead of marble statues, the Senate bean soup instead of the Senate. And while our legislators may be the most visible expression of the city, many residents would argue that Bob's Oreo Ice Cream is the truest.

Following are some of the suggestions from Kitchen Bazaar customers--residents' recommendations for taking your tourist-season guests on a food tour of Washington:

* Fried dumplings from the Szechuan and Hunan Restaurant, 10 pieces for $4.25, 1776 E. Jefferson St., Rockville.

* Thai Room Special Chicken, chicken drumsticks stuffed with crab and covered with a spicy sauce, $3.75 at the Thai Room, 5037 Connecticut Ave. NW.

* Hunan Green Beans, $5.50 from Hunan Garden Restaurant, 2104 Veirs Mill Rd., Rockville.

* The Peking duck, $18 for two at the Peking Gourmet Inn, 6029 Leesburg Pike, Falls Church.

* Pasta Pesto, a cold salad with corkscrew noodles, mushrooms and red peppers at $5.50 a pound from Fete Accomplie, the gourmet take-out at 3714 Macomb St. NW.

* The white pizza from Amalfi Ristorante Italiano, 12307 Wilkins Ave., Rockville; small $3, large $6.

* Thai Fish at Chez Wok of Georgetown, deep fried and covered with a carmelized onion and ginger sauce and served as part of a buffet where $5.95 buys the main course, rice, bread and butter, wine, dessert and coffee. The fish is not always on the menu, so check first with Chez Wok, 1015 Wisconsin Ave., NW.

* Soft shell crabs at Chez Grandmere, 3057 M St. NW. They should be on the menu any day now, but call to be sure.

* Steamed crabs from the Chesapeake Crab House, 8214 Piney Branch Rd., Silver Spring. Depending on size they are $8, $12 or $16 a dozen to go and $9, $13 and $17 a dozen to eat there.

* Crab imperial from Crisfield, 8012 Georgia Ave., Silver Spring, where it's $15.75.

* Warm green bean salad with chicken cracklings and appetizer included in Mrs. Simpson's prix fixe, three-course dinner for $15. Mrs. Simpson's, 2915 Connecticut Ave. NW.

* Crusty peasant bread, $1.30 a loaf, at Bread & Chocolate, 611 King St. or 5189 Leesburg Pike, both in Alexandria.

* Biscuits with raisins, $1.09 per half-dozen, from Posin's, 5756 Georgia Ave. NW.

* Rum buns from Hogate's, 9th & Maine Ave. SW. "Yucky and sticky but terribly Washington," says a fan, but also only available when you order a meal.

* Oreo Ice Cream from Bob's Famous Ice Cream, 2416 Wisconsin Ave. NW.; $2.50 a pint, $4.75 a quart.

* Chocolate chip cookies from the Cookie Connection, $4.95 a pound, and, says the source, irresistible when two form a sandwich around a scoop of Haagen-Dazs ice cream ($1.50). In the subway stop at 1101 Connecticut Ave. NW.

* Chocolate chestnut cake and/or hazelnut cheesecake, $3 per slice, at Suzanne's, 1735 Connecticut Ave. NW.

* Mocha cream cake, $6.50 and up, from Van Tol Bakery, 2516 University Blvd., Wheaton. Must be ordered in advance. Bakery open Friday-Sunday.

* Maple walnut or macaroon bisque ice cream from the University Pastry Shop, 3234 Wisconsin Ave. NW, $2.40 a pint or $4.50 a quart.

* Chocolate cake at 209 1/2 Restaurant, 209 1/2 Pennsylvania Ave. SE, $3.50 a slice or, $50 when you order the whole cake in advance.

If you'd like to watch Calvin Trillin eat, or have him sign his new book, you'll find him at the Kitchen Bazaar, 4455 Connecticut Ave. NW, on Thursday from 1 to 2:30 p.m. and 7 to 9 p.m