Southern Mexico itinerary - Mexico City to Cancun

Hi all.

I've got a rough plan for a month long trip in southern Mexico and wanted to know if you've got any suggestions for interesting places along this route that I might have missed in my plans. By rough I mean the days in each place are completely flexible to be used as a guide, the route is also flexible. All travel will be by bus, and in between each stop I've included a day for travel. The activities I've included at each stop should give an indication of my interests (photography, diving, community based cultural/history tours/lessons) to make it easier for any suggestions you might have. I hope to stay in more locally owned B&B type places along the way so I can practice my spanish.

The short days in Campeche and Tulum are because I've been there before and I'm simply showing my travel buddy around. I'm arriving late October next year in time for day of the dead festivities in November. I Understand you need several months to properly explore Mexico, but this will be the last leg of a 5 month trip through North America and Cuba so no real desire to stay longer than a month.

Mexico City - 4 days (art galleries among other things)
Oaxaca - 4 days (art..)
Sierra Norte - 3 days (hiking)
San Cristobal - 4 days (Chamula day trip, markets)
Palenque - 1 day (ruins, waterfalls)
Campeche - 1 day (architecture photos)
Tulum - 1 day (ruins)
Cancun (general area including islands) - 4 days (diving)

(28 days in total including 1 day travel time between each stop)

If you had any great experiences with community run tours in Chiapas or Oaxaca please share. I Appreciate any advice you have. Many thanks, from Aus :)