There is a long stretch with not much to see leading into downtown. There is an occasional restaurant (Farandula, Pepe's) and one well-known discotheque (La Boom, whose techno beat was not to my taste), and on the right it is mostly golf course.
You've arrived in downtown, at the bottom of the main street Avenida Tulum,
when you see "Rolandi's" restaurant on the right.
From here you proceed on foot northeast along Tulum.
Along the right side are flea markets and street vendors.
Along the left side are conventional stores and restaurants.
When you come to Avenida Uxmal (the first complicated intersection),
you've just about reached the end of the interesting part.
There are a couple more blocks on the right where you'll find
the giant "Commercial Mexicana" store and some cinemas, but after that
it gets seedy.
Walking along the main drag, like Dante I turned left when the opportunity arose. Beware that you have to go past several minor streets, and even sometimes cross park-like things, to get to the next interesting street which is Avenida Yaxchilan. It runs parallel to Tulum and you will find tons of neat little restaurants, where the greeter will offer you free beers and margaritas to come in. There are also some nice small shops along this street.
An interesting street that branches off from Yaxchilan is Avenida Sunyaxchen. I didn't go very far along it, but it appeared promising. This is where the downtown location of the "100% Natural" restaurant is located, just past the intersection with Yaxchilan, on the left-hand side. All of which leads me to conclude that the best streets in downtown are those that have "yax" in their names. :-)
I didn't find much to see along Avenida Uxmal, which runs across both Tulum and Yaxchilan. There are a few stores if you are interested, and a Kentucky Fried Chicken if you're a fast-food junkie.
Avenida JC Nader runs parallel to Tulum on the opposite side. I strolled a little ways down it but didn't see much; one side was mostly residential.
Avenida Coba crosses Tulum and Yaxchilan in the middle of downtown. Although I didn't find much in the way of shopping along it, I did find it one of the more picturesque residential areas and explored some of the side streets that branched from it.
Avenida Bonampak I didn't see at all; on this street is the Plaza del Toros where the bullfights are staged each Wednesday.