A covered market with everything - cowboy hats, huge roses, fresh fruits and vegetables, tamales, christening dresses that look like wedding cakes and tons of shoes. If you don't speak any Spanish at all this won't be any easy place for you, especially if you like to haggle on prices.
I was in there every morning buying flowers - and for the Day of the Dead there was an extra large flower market set up on the streets surrounding.
Advice - don't eat the green oranges - they make great juice but don't taste very good. If you are dying for a taste of fresh fruit or a salad you can buy water treatment tablets at all the grocery stores. You put the food in water (tap or bottled), add some drops and let it all sit for 20-30 minutes.