Hi Anne-Laure,
San Francisco has some absolutely amazing urban projects. You should visit as many parklets as you can. Here is a Google map. Walter Hood’s Powell Street Promenade is pretty cool (super central, you won’t miss it), I love the TransAmerica Redwood Park, and Levi’s Plaza Park is one of my favorites.
I actually did a trip a little while ago and documented the places I saw (I have the addresses for each park too). You can see them here: http://landscapevoice.com/tag/san-francisco/
Los Angeles also has some great gems. I didn’t get to visit as many places as I wanted to since it’s such a spread out city and I didn’t have access to a car. You can see some of my favorite urban projects from that city here: http://landscapevoice.com/tag/los-angeles/
Hope that helps!
Lucy