Piedras Marcadas Canyon is part of the Petroglyph National Monument. The trail is fairly short, 1.5 miles and wanders in and around hundreds of petroglyphs. It is a day use area only so make sure you arrive before it closes at 5pm. This part of the National Monument is within the city limits but it is a very nice hike and the petroglyphs are just amazing to see. Boca Negra also has many, many glyphs as does Rinconada Canyon. Awesome to see and nice, fairly easy hikes as well.
Note: Rinconada Canyon was recently closed due to the collapse of the trail.