It won't be long now until you have tried the new crop of restaurants that hve sprung up around your office or home while you were on vacation, so for the next time you get bored looking for a different lunch spot, here is a new look at three old places that do something special for that meal.

Madison Hotel Coffee House, Le Rendezvous and Retreat Bar, 15th and M Streets NW. 862-1600. Mexican dishes served weekdays, noon to 2 p.m. or as long as they last. All major credit cards. No reservations in coffee shops. Prices: Mexican main courses average $7.50 to $8. Regular menu also in effect.

Mexican food is heavy, right? Mexican food is cheap, right? Mexican food is heavy and cheap and fire on your tongue, right? Wrong. The Mexican food being served at lunch in the Madison Hotel's coffee shops and bar is delicate and light, full of subtle complexities, only sometimes hot.And, like most things at the Madison Hotel, it is more costly than almost anywhere else in town.

Every weekday at lunch there are four or five Mexican main dishes. In addition to the familiar tacos, tamales, chiles rellenos and chile con carne, the menu might include chicken with mole, Mexican or jalapeno suace; fish a la mexicano; empanadas or eggs rancheros. You can call to find out what's on the day's menu, and fill out your meal from the regular menu.

Classic coffee shop design, a sunny window seat in a streetside cafe or a dim bar are your environmental choices. The waitresses are institutions here, having enough experience to handle anything without batting an eyelash. Once you become a regular at the Madison, your tuna sandwich is likely to hit the table before you havae your napkin in your lap. Even on busy days when they get you off to a slow start, they seem to make up for it by meal's end.

Don't expect hearty portions on the Mexican menu. The two empanadas ($5) are delicate in size as well as so light and flaky that the dough must have hardly been handled in the making. The ground meat, olive and raisin filling is hot enough, but leaves room among your senses for a little playfulness. The tamales are also small, served with the husks already removed, but are the lightest and freshest tasting cornmeal dough I have encountered. More substantial meals are chiles rellenos -- the light, eggy crust not at all greasy, the filling an intriguing mosaic of melted cheese and ground chicken. With it are light, bubbly-crisp fried strips of tortilla covered with sour cream and guacamole.

The guacamole, though different each time I tried it, is always superlative, whether it is predominantly seasoned with coriander or lime on a particular day. The refried beans are smooth, mild, quite good; only the rosy-tinged rice is uninteresting. Chili is sometimes available -- without beans, but crusted with cheese -- a dark, meaty version. And the standard dishes, the tacos and enchiladas, are well above the crowd. Alongside each dish is some sauce or other: fresh tomato, cooked tomato, tomatillo, each of them peppery but the green tomatillo absolutely explosive.

If you want to do this right, of course you order a Coke. Served in the bottle. Well, maybe a beer.

Sunset Restaurant, 920 11th St. SE. 547-9626. Monday through Saturday, 9 a.m. to 2 a.m. No credit cards. No reservations. Prices: Main courses with two vegetables $2.75 to $4.50; sandwiches 85 cents to $2.25.

I had been told that the Sunset sold the best crab cakes in town. That is not true, but I can still believe that it sells the best $2.25 crab cake sandwich in town, the big triangular crab cake with filler but not too much of it and shell but not an outrageous amount, tastes mostly of good, fresh crab. And for your $2.25 you also get nostalgically correct handmade coleslaw faintly sweet and still crisp.

Sunset is an in-town roadhouse, a crner joint under the expressway with wood-like paneling and booths, sparkling beer signs and beeping pinball machines. It is the kind of corner bar where nearly everyone is wearing a hat -- baseball, fishing, hunting, whatever -- and pistols are bulging from an astonishing number of shoulders and waistbands.

You are safe ordering the homemade soup and the potato salad, which is embellished with pickle relish and celery seeds. The hamburger costs all of $1.45, big, lean and spatula-flattened, tender because it was lightly handled and not packed too hard.

Sunset serves breakfast, lunch and dinner, though not much is likely to be left in the kitchen by dinnertime. The menu is a parade of Americana, from liverwurst to ham and cheese to pork chop sandwich and fried chicken to salisbury steak. It is, however, the special of the day that is a knockout buy, maybe a mountain of fresh chicken salad with potato salad and slaw on the side for $3.25. The Sunset does not get so intense as to peel its own potatoes for frying or bake its own pies, though the waitress may be inspired to make a cake on occasion. It serves homemade food -- up to a point. And there certainly is nothing fancy. But it is among the last of a breed.

And (the final test for any luncheonette that has passed the homemade soup hurdle) the Sunset serves terrific, full-strength iced coffee. For 35 cents.

Sheraton Carlton Hotel, 16th and K Streets NW. 638-2626. Vegetarian buffet lunch, Saturdays, noon to 2:30 p.m. All major credit cards. Reservations. Price: $10.

Vegetarians no longer need to feel like second-class citizens, picking among bland vegetable plates or filling up on grilled cheese sandwiched. The Sheraton Carlton, once a week anyway, dishes up glamour, vegetarian-style.

Even a grilled cheese sandwich would be better for being eaten in such a dining room, its high ceiling a treasure of painted wood beams, its white walls set with arched windows and imposing early American portraits. It is a glorious room, and the buffet table is appropriately appointed with silver chafing dishes.

After soup is served, you are on your own to fashion your meal from several buffet tables. A table of salads includes crudites, marinated cauliflower, artichoke hearts and green beans and waldorf salad. At another table, if you can rouse the cook, you have a choice of omelets made to order. The hot table curves from buttery fresh green beans amandine to excellently seasoned spinach crepes to fried apple rings. Along the way are wedges of dreadful spongy and pasty pizza, plus potatoes, zucchini and carrots that are nothing special. Special on that main table are fresh fruits: pineapple, figs, melons and strawberries.

Vegetarianism is no longer inextricably identified with self-denial, and the Sheraton Carlton takes note of this with its dessert table, two enormous silver bowls, one with strawberries romanoff, the other with syllabub, a custardy whipped cream laced with slivered orange peel and orange liqueur. They are sumptuous endings.

The glittering formality of the buffet is sabotaged by the service, which is homespun and prone to mother-hen clucking, but neglects to remove plates between courses. Waitresses are likely to rest their trays on your table while they chat with you. The amount of attention is commendable, but the service just seems out of character.

In all, the vegetarian buffet is mixed, with enough to please once you have weeded out the worst. But never has a vegetable garden been planted in a grander setting.