Located about a third of the way to Culiacán from Mazatlán is the beach site of Las Labradas. The site features hundreds of petroglyphs carved into the rocks on the beach, and is a fascinating day trip from either Culiacán or Mazatlán. If you’re without transportation and want to spend a few $$, you can take the tour from Onca Explorations out of Mazatlán. Strangely, the tour isn’t as popular as one might think, so you might need to round up enough people to meet the minimum (it was four when I did the tour).
This is definitely a trip to make if you find yourself in the area as there’s nothing like it anywhere else in the world. Another MéxiWOW experience!