Plaza Grande is the heart of Merida's central historic district, and the hub of its cultural activities. This vibrant and colorful park, situated by the iconic Catedral de Merida San Ildefonso, is a short walk from many other prominent landmarks and historic sites, as well as theatres, museums, fine restaurants, and specialty shops. It's also the site of street vendors and free performances and events. One of the most intriguing is a regular enactment of Pok Ta Pok, a ball game played by the ancient Mayans and playing great ritual significance in their culture. The game and the ceremony around it are fascinating, and while the Pok Ta Pok game is an amazing display of skill, what's even more impressive is that it's followed by another game played with a ball of fire.
If you go:
The Pok Ta Pok games are every Thursday at 8:00 pm. Admission is free, but if you want a seat, and thus a good view, you will need to get tickets the day before. These are available at the Olimpo Cultural Center, on the other side of the park, Or if your party consists of no more than 2 or 3 people, you may be able to get a seat after other people leave early. Be sure to take the free walking tour of Merida to learn more about some of the sites in this vicinity and the city itself.
Links:

Calle 60 South/ North
Centro
Merida, Yucatan 97000 Mexico