I am considering taking a trip to Mexico, perhaps taking in some of Guatemala too, in January. However, we only have 2 1/2 weeks. Is it worth it for that short time and if so can anyone suggest a good itinerary/places to see. (I've been to the yucatan peninsula in the past and would like to go somewhere new...)
Many thanks!

Since you have been to the Yucatán Peninsula and do not rule out more time among the Maya, here are my suggestions:

Fly into Mexico City. Fly on the very next plane to Tuxtla Gutierrez. You could book ahead, but the price right at the ticket counter on the same day has been quite a bit cheaper in my two experiences. Spend night at Tuxtla. I think the regional museum is well worth a person's time and others swear by the Sumidero day trip, but I have never done the latter, so I can't say about it.

Bus to San Cristobal, mostly for day trips (Chamula and/or Zinacantan), and seeing Na Bolom.

Bus to Palenque. Stay at El Panchan, which I loved for the many young tourists from many lands who put up me. Panchan is right at the official entrance to the park, is inexpensive and one of the hotels has Internet service. See the ruin site and be sure to explore some of the trails around it. Don't miss the museum; they have really spruced it up.

Combi van to Frontera Corozal early one morning from inside the town of Palenque. On arrival, meet up with others to share a boat to Yaxchilan. This is four hours, costs around $50 USD for the boat and boatman, which includes two or so hours at the site. The rest of the time you will be able to see and enjoy the Sacred Monkey River, the Usumacinta, as your boat chugs along there and back.

Leave Corozal ASAP, unless it is late in the day, in which case, the Hotel Escudo Jaguar is overpriced, but okay. There is at least one basic hotel near the small museum with little to see in it. The only place I could find with cold beer to go was Cortez's tienda, which you can easily find, but there was no aguardiente and attempts by an impromptu group to play Hearts got shut down around 9 PM. Consider spending the night at the hotel in Bethel.

Head on by boat and bus to Flores. See Tikal, but stay in Flores and enjoy the wonderful sunset from the Sunset Cafe or the Hotel Peten. See Uaxactun.

An additional option: Bus to Sayaxché. Stay in Hotel Guayacan. See El Ceibal and arrange a jungle day trip to Dos Pilas.

Return to Chiapas. Fly from Villahermosa to Mexico City. See Teotihuacan. See Zocalo and museums.

Fly home.

Divide this all up as you go along and as things look interesting or not

Have a great trip!

I should mention that if you have other questions, post them and I will see them and try to answer.