Around the central park there are sellers who show the typical Mexican white dresses with beautiful coloured embrodery. If you want to photograph people, ask them the permission. Some people think that your camera could steal their soul.
Valladolid has a great zocalo. There are plenty of benches and "lovers chairs" (two chairs made to face each other) for people to sit in. Many people hang out in the zocalo and just sit. It is a great way for people to spend time with one another. While sitting in the zocalo myself, I saw couple canoodling, grandparents spending time with their grandkids, men talking about life and the weather, men getting their shoes shined, people on their laptops, and people just enjoying life. Do like the locals, have a seat and spend some time in the zocalo.