Hey mclarjh, that sounds appealing - but is three days enough for Mexico City? We're thinking of doing a day trip or two while we're there.
And thanks for the Valladolid suggestion, ref_traveller - happy to bear that in mind.

Hey mclarjh, that sounds appealing - but is three days enough for Mexico City? We're thinking of doing a day trip or two while we're there.
And thanks for the Valladolid suggestion, ref_traveller - happy to bear that in mind.
Well, last winter I spent three weeks in Mexico (not including weekends, when I traveled out of town). And I´ve already visited many times. So there is no correct answer. There is a lot to see in Mexico City, but I think first time visitors can also find it intimidating. So a few days will just give you an introduction to the place. Same with the other places you´re visiting; one or two days will just be enough time to sample these places.

Thanks everyone for the input.
There are a few other restrictions like internal flight prices (Cancun to Mexico City early in the new year is really expensive) so we're opting for:
Merida - 2 days (then overnight bus)
Palenque - 2 days (then direct flight)
Mexico City - 4 days (then direct flight)
Valladolid - 2 days (then local bus)
Tulum - 2 days (then home)
So two internal flights and only one long bus ride. We'll leave Oaxaca and San Cristobal for another time.
Looking forward to it!
Are there really direct flights from Mexico City to Valladolid, or will you fly into Cancun again, or Merida?
Lots of places to stay in City of Palenque. but booking in advance on holidays is recommended.....Its a short drive to the ruins which are a "Can't Miss"...You can stay closer to the ruins at a few smaller places...
I did the day bus from Merida to Palenque...not bad.
Mexico City is worth 5 days...Anthropology Museum has major artifacts and the Pyramids out side of Mexico City are worth an entire day...A lot to see there that rushed tourist buses wont give you time for. The Rosa Bar District is Great...I stayed @ Zocolo Hostel and took the "sight seeing bus" every where...
Taxis are safe but take a newer one with good plates & set the price in advance...
Going to Frieda & Diego House is a Taxi Ride to a suburb...Then Walk to Trotsky House from there...
Buses are cheap and few longer that 8 hour ride to most places...Might want to fly back to Cancun if that's where you started...
The Metro (5 MXN) and MetroBus (6 MXN) serve most of the tourist attractions in Mexico City.
Is Palenque important? I'm asking because while the ruins are very nice, there is really nothing else worth seeing anywhere near it. (The two waterfalls nearby - Agua Azul and Misol Ha - are not places I would bother to see if I only had 2 weeks in Mexico unless I've never seen a waterfall before.)
I guess you could do a tour to Yaxchilan and Bonampak from Palenque to fill a second day, but I would not go to Palenque unless I also included more places in the state of Chiapas, like San Cristóbal de Las Casas or Comitán. In other words, I wouldn't visit the state just to see Palenque. It's only going to help you break up the trip if you're taking buses and it's on the route between the places you want to see. If you're mostly going to fly between destinations anyways, then it's really not going to help in that department - it's just adding an extra flight.
In your place, I would probably limit the trip to a few places around Mexico City - whatever interests you: about 3-4 days to visit places within the city, another day for a trip to Teotihuacán, maybe a couple of days in Taxco - and places around Cancún - such as Tulum, Chichén Itzá, Valladolid, Mérida, Uxmal, Campeche and maybe even Calakmul. I would save Oaxaca and Chiapas for another trip. There are enough things to do in those places to easily fill another 2-3 week trip on its own.
Yeah, this itinerary includes a lot of extra effort just to get to Palenque... which is amazing, but also makes less sense if you're not going to include the nearby and equally extraordinary SCLC.

Thanks for your input both.
We like the look of the forest/greenery around Palenque and found a good-looking hotel (El Colombre) near the ruins for pretty cheap. We're also into waterfalls and wild swimming - so we're thinking this could be a couple of particularly rural days, with the rest of the trip spent staying in cities (albeit doing day trips to places like Teotihuacán, Chichén Itzá).
San Cristobal looks good but seems too far for a day trip from Palenque? Yaxchilan and Bonampak both seem to be 2-3 hours away though so there's good potential to see one or even both on the second day.