Chicago is called the Second City, but it has its share of No. 1's. Among them are the oldest commodities and futures exchange, the largest marketplace for perishable commodities and the tallest building in the world.

In fact, three of the world's tallest buildings are here: the Sears Tower, at 1,454 feet the No. 1; the Standard Oil Building, 1,136 feet, and the John Hancock Center, 1,127.

Even though it's not the tallest, the 100-story Hancock (at Michigan Avenue and Chestnut Street, just north of the Loop) is tops in creature comforts. The view from the bar on the 96th floor may be no better there than in the observatory two floors below, but there's no admission charge (it's $2 in the observatory) and more incentive to linger. If it's clear, you get a view comparable to that from a jet beginning its descent and can see lights twinkling across the lake. There's a restaurant on the 95th floor for those who absolutely can't tear themselves away.

When you come down to earth, you could spend a week in the city's museums and galleries.

Highlights at the Museum of Science and Industry (Lake Shore Drive at 57th Street) are an operating coal mine and a German submarine. Admission is free.

The Field Museum of Natural History (Roosevelt Road and Lake Shore Drive) includes exhibits of ancient Egypt, prehistoric man and Indians of the Americas. Admission is $2 for adults and $4 for a family.

The Art Institute (Adams Street and Michigan Avenue) is one of the nation's finest. Among its most famous paintings are Seurat's "Sunday Afternoon on the Island of La Grande Jatte" and Picasso's blue "Old Guitarist." Also, there's "The American Windows," stained glass homage to American culture by Marc Chagall, a trading room from the Chicago Stock Exchange done in 1894 by Dankmar Adler and Louis Sullivan, and the 67 Thorne miniature rooms.

If the trading room at the Art Institute interests you, you might want to get the big picture at the Board of Trade (141 W. Jackson, at the foot of LaSalle Street), the world's oldest commodities and futures exchange. There's a visitor's center with a view of the main trading room and guides to answer question. No charge.

The Mercantile Exchange (444 W. Jackson) is the world's largest marketplace for perishable commodities. If that doesn't impress you, take a look close up; the visitors' gallery is open and free.

If you aren't satisfied with the animals at the free Lincoln Park Zoo (100 W. Webster), head out to west suburban Brookfield, where the animals' habitats are nature-like and moats keep you at a comfortable distance. Except on Tuesdays, when admission is free, the cost is $2 for adults.

More information is available from the Chicago Convention and Tourism Bureau at (312) 225-5000. A Visitor Eventline has the latest on what's going on; dial a tape at (312) 225-2323.