This brick patio with shade (more when the trees have leaves), and a rare patch of green grass, are just northwest of the Tucson Museum of Art — part of its Historic Block. The easiest access is through the museum's free parking lot (though the lot is for museum visitors only; there's plenty of street parking in the area).
On the wall below this spot, at the corner of Main and Washington, is a plaque marking the northwest corner of the Presidio:
Location: Downtown, at the southeast corner of Main Avenue & Washington Street
Parking: Street parking
Hours: None posted