One place that made the news recently is between the Leo Rich Theater and the Music Hall — just south of La Placita and north of the Tucson Convention Center. The metal tractor seats that lined the area have been replaced by benches and trees have been planted, as you can see in this video from City of Tucson 12:
Location: Here's a satellite view from Google. (Click the + button to zoom in.)
Hours: Mid-day in the summer is scorching; there's no shade (yet).
Parking: Street parking and the two TCC lots are both about a block away. Try walking from downtown over the Garces footbridge (which crosses Broadway and Congress).