There is a conspiracy innocently spread abroad in Washington and its environs these days that contructively entertaining children daily during the summer is (a) expensive, (b) done by professionals and (c) the job of institutions outside the home.

Many private schools have day camps; museums, nature centers, parks and churches all provide programs for young children. Our 5-year-old daughter has been solicited this spring by day camps specializing in soccer, ballet, religion, swimming, arts, nature and theater.

Such arrangements for young children are an expensive luxury, especially when registration forms begin arriving just as April taxes are due. With some advance planning, a liberal dose of cynicism for institutional solutions to family dilemmas and a pinch of nostalgia for childhood in the '50s, summer vacation can be inexpensive and -- maybe -- as enjoyable as day camp and mother's helpers for kids and parents alike. Here are eight strategies aimed at keeping spirits high and costs low.

Acknowledge that boredom exists, and is an acceptable emotion.

It wouldn't be summer without vivid and frequent complaints of boredom. In the heat, tempers have short fuses. But children of the '80s have as many inner resources for constructive self-amusement as we seem to think we had in the early '50s. We had yo-yos and jacks, made clover chains, played monopoly ad nauseam. We used to sit on the front stoop and count cars at the end of the day, make up bizarre stories about strangers who happened to walk by, and bet on which car would be Daddy's.

At the risk of sounding medicinal, every child needs a dose of boredom now an then to appreciate how good it feels to occupy oneself. Everyone needs to learn to manage time, loneliness, leisure. (A good source of boredom-beaters is Sid Hedges' Games for Children While Traveling (Grosset & Dunlap, 1973).

Plan a project that cannot be finished all at once .

One summer in the '50s my brother built a radio on a table in a corner of the porch. Daily for an hour he studied incomprehensible diagrams, arranged and soldered little thingamajigs. The smell of solder is my equivalent of Proust's madeleine -- one whiff and it's Quincy, Mass., in the '50s again.

My inviolable table in the opposite corner contained a "Visible Woman" kit; I glued and painted organs and bones all summer long.

Accumulate usable junk .

Radio-building and anatomical models are not suitable, of course, for younger children. For our 5-year-old we gathered smooth, hand-sized rocks for a rock-painting project. She intends to wash and paint a tray-full and have a friend with a housewares/gift boutique market them.

Last summer the then-4-year-old (and I) build and furnished a doll house from egg cartons, strawberry baskets, cellophane wrappers and fabric scraps which we begun accumulating in early spring. The doll house is now in the attic and my daughter plans to do one each summer until we have a neighborhood stashed away in the attic.

Such projects generate their own energy. Library visits include books on doll houses; neighborhood walks, taken a thousand times already, become new when we talk about building materials, door construction, roof and window styles. The younger builder becomes a better observer.

Egg-carton constructions are documented imaginatively in Egg Carton Critters (Robert Dunne and Donna Miller, Scholastic Books, 1978). Regrettably, no museum in Washington has yet taken junk to the heights available at the Boston Children's Museum, which fills part of its gift shop with barrels of child-safe industrial scraps.

Have a garden for each child .

The gardening mania has reached the Oshkosh B'Gosh set for all the right reasons. A one-yard-square raspberry patch entertains our 2-year-old as well as 30 minutes of "Sesame Street." A handful of raspberries, warm from a long summer afternoon, hunted by greedy eyes, makes a special appetizer with raw snowpeas . . . which never make it into the salad bowl.

An iris lends itself perfectly to shadow drawing, an art form I thought I invented in a Quincy garden in the '50s. The shadow of a strong flower can be traced perfectly and then colored by the young realist.

Caterpillar and ladybug races are daily events at our house these days. A delightful -- and deep -- book about children and insects is Cricket Boy: A Chinese Tale (retold by Feenie Ziner, Doubleday, 1977).

Watering, picking and weeding one's own produce, cutting and arranging one's own zinnias, reinforce the child's bond -- too easily lost -- with the earth and other tender growing things.

Plan one field trip per week .

An early-morning visit late in the week to the National Zoo (feeding time is at 9 a.m.) and a 10:30 picnic lunch give a form to the whole week. Another week it is a supper picnic downtown near Daddy's office and an evening visit to a museum. (Summer hours at the Smithsonian extend to 9 p.m.)

For days before the zoo trip, the playdough on the child-sized worktable becomes animals; there are cheetahs in the backyard bushes; elephant trunks -- not rubber hoses -- water the garden. Young cooks shop for the supper picnic, plan and pack the cheese sandwiches and watermelon.

Extend old ideas in new directions .

Last year's lemonade stand is this year's gypsy fortune-teller's booth on our corner in Amreican University Park. Young gypsies work for an hour in advance, arranging their wardrobes, wrapping their crystal ball (tinfoil on a grapefruit) and rehearsing their spiels.

Sheet-covered lawn chairs become desert oases; new images vivify old stories. At the library, for example, we have to find Ludwig Bemelmans' Madeline and the Gypsies and collections of old tales.

Invite company often and at various times of the day.

We regularly schedule a small outdoor luncheon party, tea party or favorite guest for dinner. The children can participate in every stage, from shopping to cooking and serving, but probably not cleanup. (One can't have everything.)

Set low goals for the adult in charge .

Last summer I swallowed the urge to list weekly goals -- long and short-term, creative, decorative, intellectual. Instead my one ambition was to defrost the deep freeze. No list of books to read, no research project to begin, sort of complete. No box of sewing. No piano practicing, no stack of photographs to sort.

Somehow the children were surprisingly generous with private time for me, and books were read, fall sewing begun and -- mirabile dictu ! -- my office was straightened, and lesson plans for a new year of teaching Latin were underway before Labor Day.

Needless to say, with such a regimen this summer, we will save money which we immediately will spend on boat rentals and meals out when we do go on a family vacation. In Washington this summer, there will be many days when we never turn on the television, never run the automobile, never set foot into a store. And the return to school in September will be both bitter and sweet, but not just more of the same.