Urban-design nerds, street-festival fans, and earnest neighborhood do-gooders will converge on a block of Holman St. near LaBranch this Saturday to play a little game of make-believe: They’ll be imagining what it might be like if Houston had some sort of street life. From 10 am to 10 pm, they’ll be hanging out on a Third Ward Midtown block quickly made up to look as if it did: with painted-in bike lanes, instant street trees in planters, a pop-up cafe in a portable building parked on the street, food trucks, and maybe even a farmers’ market. And they’ll be smiling pedestrian-friendly smiles.