In the past five years, six unusual public spaces have opened in downtown San Francisco as spin-offs of large development projects. They join 15 others that were created after 1980 to provide breathing space for employees and visitors during the week. The map does not include earlier corporate plazas or free-standing public spaces, such as Yerba Buena Gardens.

Why you should go: Protected from the wind, 11 stories off the ground, this three-story-high box combines views of the traditional city with a sci-fi ambiance. Two pedestrian bridges allow you to shift perspective, from wide-open command of the bay to focused perspectives on Civic Center through blades of opaque glass. Lots of comfortable seating as well.

Why you haven't gone: How would you know? Nothing on the building's exterior indicates that casual visitors are allowed, so architecture junkies tend to be the only ones who wander inside without an appointment. Worse, this federal outpost is at a security level one short step below airports; be prepared to take off your shoes after the guard inspects your ID.

What can be done: Once inside the door, the government provides a flyer that directs you to the Skygarden. An information kiosk outside would be much more helpful. And the shoe thing seems a bit much.

2 Courtyard at the Infinity 300 Spear St. Why you should go: One of the city's most artistically playful nooks threads residential towers between Spear and Main streets - a bulbous landscape of green hillocks, each topped by a single deciduous tree. Black granite steps double as sawtooth seating that invites you to lounge; a discreet fountain along one wall provides that ever-so-soothing white noise.

Why you haven't gone: This terrain designed by Hargreaves Associates isn't required by the downtown plan, so no sign invites us in. Nor is there much to see beyond shaggy hillocks and sleek glass. By contrast, the large terrace at Hills Plaza to the east has a wide-angle view of the bay.

What can be done: Not a whole lot, short of putting musicians and free food on the granite steps. Folks do pass through; as the neighborhood develops, there will be more.

3 Enclosed plaza at Millennium Tower

301 Mission St.

Why you should go: Inch for inch, this 26-foot-high space is one stylish oasis. The floor is dark green stone. The wooden benches are well-crafted and thick. The rear wall doubles as a glass mosaic rendition of a forest by artist Amanda Weil. Along with style comes convenience: a restroom on your right as you enter from the street.

Why you haven't gone: This space pulls a disappearing act even though it's smack along the street, dimly lit behind reflective walls of glass. Large stretches of the Weil artwork are indiscernible, for instance, because there's no light behind it. On the odd chance a passer-by does notice, what do they see? A dark empty room.

What can be done: A coffee kiosk at the angled entrance would help. So would backlighting to Weil's installation.

4 Terraces at the Intercontinental Hotel

888 Howard St., fourth and sixth floors

Why you should go: Peace and quiet and afternoon sun. The sixth-floor space includes plush lounge furniture that is waiting for a pool to magically appear.

Why you haven't gone: No reason to go. The story is that the City Planning Commission insisted these spaces be made public, even though the developer already had chipped in funds to create nearby Mint Plaza. Wind caroming off the building to the north gets so brisk, the fourth-floor terrace now sits in a plastic tent.

What can be done: Food trucks lifted by crane onto the terraces at lunch?

5 Roof Terrace at One Kearny

Why you should go: There's no better view downtown for free. Besides the sweep of Market Street, a telescopic vista down Third Street culminates in the bay. To the east, a collage of towers offers a crash course in the past century of architectural styles.

Why you haven't gone: Five "no trespassing" signs along the building's Market Street frontage, but no hint of a public terrace. The sign by the door does a disappearing act. Being forced to sign in will scare some people off. Gee, maybe you aren't wanted.

What can be done: For starters, post at least two signs outside - and in metal not glass. A duplicate of the plaque found in the elevator, for instance.

6 555 Mission St.

Why you should go: The one conventional newcomer includes a variety of seating opportunities, a luxuriant planted wall, Blue Bottle Coffee around the corner on the alley, and eye-catching artwork to stop you in your tracks: a trio of amiably absurd aluminum heads by Ugo Rondinone and a pyramid of bright metallic humans by Jonathan Borofsky.

Why you haven't gone: Because you haven't walked by. This is as inviting as they come - not a destination, just an easy place to kill some time.