At the Fifth Floor Galleries, New City Hall. Free.

THE PROJECT '70 Exhibition features designs and crazy schemes dedicated to the un-American ideal that cities should be full of good things to look at and play with. Not an assembly of finished works, the exhibit includes plans and models designers have made in the six months since Project '70 was first organized by the Institute of Contemporary Art, Final, full-scale projects should be sprouting all over Boston by December.

For these sculptures, murals, playgrounds and unclassifiable opuses aren't meant for millionaires' collections or 9-5, open-late-on-Tuesdays museums-they will enliven streets and public places. And perhaps because anyone from grade-school children to established artists could submit a project, designs run a never-stuffy gamut from the mischievous to the elegant.

Some participants worked out structures that use simple means to change the very mood of a city-to introduce new functions and fulfill traditional ones in more exciting ways. Neil Goodwin and John Borden, for example, designed a system of mobile vending booths because "... Government Center, the Southeast Expressway, and other 'urban renewal' in the West End of Boston have resulted in the devitalizing and deadening of a uniquely alive part of the city." Their hexagonal booths might be set up in under-utilized spaces like City Hall Plaza to dispense goods and information. Such units could provide homes for the kinds of marginal businesses usually wiped out by urban renewal.

These booths are cleverly designed. Their over-hanging roofs lower to cover the (smaller) bases for overnight and bad weather storage, incorporating display, storage and roofing functions in one compact unit. The booths' unusual form would become recognized as a signal for their function-and this kind of visual guide can make a city more legible and therefore more comprehensible.

A wilder urban system is Dorothy Proctor's plot to install a love-in on every park bench. Her life-sized fiberglass grandmas, boy-with-dogs, and hand-holding couples may be seated throughout Boston. Each sculpture would leave room for six live people on a bench. So, according to the artist, "You could put your head on a girl's lap or lean against Grandma, and a child could pat the dog without damage to either." A less cozy proposal specifices concrete benches-figures and bench would be cast as one piece.

Peter Latham would have sedentary Bostonians playing "Cracket-an outdoor game" on every asphalt parking lot. The game is a stepchild of squash, played with a wooden paddle against two portable walls. The court occupies only about 300 square feet. Interested athletes may practice their Cracket at City Hall-Latham set up a court in a corner of the exhibition hall.

Sierra Club plans for recreational use of islands in Boston Harbor demonstrate another shade of citizen-serving thought. To provide "vital contact with a natural environment which is close" the club will develop Lovell's Island for hiking, arts and crafts work and other activities. Certainly anyone who's spent a hot two hours in a Sunday-in-July traffic jam should be enthusiastic about this project.

MORE PROJECT '70 designers worked on one park, playground, or object than on a city-wide plan. But if all their designs are executed, Boston will have youth centers, parks, and playgrounds galore, all full of unique playthings, bright murals, awnings and light shows. And traditional art works like paintings and sculpture swill be done at the giant city-scale to propagate public pleasure-some billboards will even hold cheery graphics instead of motel ads.

Historians might say that the Boston Irish left their pots of gold in Gloccamorra, along with their leprechauns. But at least Manuel Crisostomo's "Neon Rainbow" mobilizes the visual magic contained in industrial technology. It will are over a Back Bay rooftop adjacent to Storrow Drive, according to the artist.

Also intended for Back Bay is Bill Jacobson's wall-long relief sculpture for the side of a building. Made of pieces like old railroad ties and used industrial lumber, its strong vertical and horizontal lines recall the rectilinear urban grid of the area. And the material corroborates the Populist sentiment that "wood is good"-a needed counterpoint for an increasingly steel city.

Some lucky kids will get to climb a bright yellow sheet of semielastic (plastic) Swiss cheese instead of a cold steel jungle gym when one Project '70 prototype is enlarged. And hip tykes shoud groove on a set of rope swings whose poma-lift type seats look like giant hot pink and dark purple onions. Should they fall, they won't have to hit hard macadam-Project '70 shows four-foot wide flooring dises of foam covered with black vinyl.

Not ones to let the over-10 set take all the kudos for playground design, the fourth graders at St. Lazarus School in East Boston planned their own "Possible Dream." Their mural-sized drawing depicts a "ride through space." which is taken on a yellow slide entered at top through an Apollo-type capsule. They also intend to throw objects at a gargantuan mouse-target who wears a sign saying "cheese please." A badminton court, and a spook house which has an octopus guarding it are among other parts of their dream.

Project '70 encouraged artists to work with community groups on projects, even offering to suggest communities to those who knew of none. Muralist Harry Freeman and photographer Wendy Snyder worked with a South Boston group to place a psychedelic mural and a photo exhibit on the clapboard side of the Church of Our Savior. And they say that "Art Etc. for Southie" represents "an attempt to bring the power of art into the streets."

Art power may seem impotent or irrelevant beside ideas like black power, women-power, or peace. And I suppose each person has his own idea of what inputs make life richer. But it does appear that all people seek visual joy, whether they are Steins buying Matisse paintings or housewives selecting fabric at Corcoran's. Project '70 demonstrates that it is feasible to offer people such pleasures in their cities, and that therefore proves that cityscapes need not be as bleak and depressing as they typically are.