I've done these sketches on my Iphone over the weekend. I got this little application that give you the ability to draw with one finger on the touchscreen.
It's quite challenging to see what you're doing when your finger takes a third of the screen. but it's fun. I started Saturday with the bus (top left), then the blue truck, the fire pit and finally downtown L.A from where I live, this evening.
You can watch a small clip of how I did these sketches here.