Cerro de la Máscara is a petroglyph complex on the Río Fuerte north of town. The engravings are usually linked to groups moving through the Sinaloan corridor centuries ago, and local accounts describe dozens of stones with more than a hundred carvings.
Access is the catch. This is not a polished museum site, so ask locally about current conditions and go with someone who knows the way. For travelers with an interest in archaeology, it is the strongest reason to look beyond the plaza.