North America: Unused, corporate public spaces converted into commercial spaces.
 

I was looking at Midtown Atlanta on Google Image View, when I came upon this development (the building housing the Iselle restaurant), which I find interesting and exciting.

https://goo.gl/maps/sBWHMPByHhfWVYZR6

I'm drawn towards it because it shows an example of what could be a new trend in US cities, and a way to revitalize places that have an excess of buildings with outdated, "tower in the park" designs. If done right, this practice could potentially create a rich pedestrian experience in areas dominated by big corporate towers with sparse street life. It could change the face of many downtowns.

I know there are plenty of examples of this in cities big and small, but are there any vibrant streets or areas in the US or Canada that have been planned in such a way that is noticeable in success?

Boston has a building that used to have a relatively substantial plaza (before) that had a commercial space with seating added that increased the street activation. After

The Atlanta example looks good, but the Boston one deals with the much larger problem...a freaking driveway to the front door. Though at least in their case, pedestrians aren't forced to walk around it as the are in Atlanta.

Back in the good ol' days, we used to call that building the Pregnant Building. I wonder what the locals call it today?

-- Why, you ask. Look at it, For Pete's Sake!

There've been a few of these in Toronto. Here a once windswept plaza was replaced by a shopping centre: https://goo.gl/maps/VPm67doUBeQRouJ78
Before: https://goo.gl/maps/aMa2AL2RzSE2akJ56

Here's a more unfortunate one, with the original intent of the Brutalist-era architecture being severely compromised by the new retail component: https://goo.gl/maps/awJSn9ypfcaaKL3z6
Before: https://goo.gl/maps/PWY3yJmoRjt8L4in8

There's been a lot of tower-in-the-park infill as well, with new towers or townhouses built on the vacant land. Eg:
https://goo.gl/maps/rco2rrzetJK1DwWE8
https://goo.gl/maps/cSJvRE95qvXVEb5e9
https://goo.gl/maps/a4R74zGtYyAXjLsJ8

Great examples, thank you.

No pics or links, but Charlotte is trying to inject a little life into a downtown that mostly consists of corporate lobbies, pointless corporate plazas, and meaningless corporate art. There's a considerable push to turn some of that into streetfront retail spaces.