How would you suggest getting from Cancun to Antigua and back to Cancun doing a loop, without having to take an internal flight, and including all the best stops on the way over 4 weeks?
Welcome to TravelBlog:-)
On our RTW we went from Antigua to Tikal (our favorite Mayan site ever) and then crossed the border by land to Palenque (another ruins) and went to the coast (including Cancun) from there.
The border crossing is very laidback, no crowds but it takes around 10h all the way from Flores to Palenque. Make sure you have money with you to exchange as there is no ATM at the Mexican border.
From Palenque you can take one of the night services of ADO directly to Cancun but I suggest you stop in Tulum, Merida or other sites If you have not seen them.
Any more questions let me know,
I went from Isla de Mujeres (the backpackers' Cancun) to Tulum to Belize and fab Caye Caulker to Belize City to Flores and Tikal, to Guatemala City (long journey through the jungle) to Antigua.
I then followed the spine of the Andes up through Guatemala, stopping lots along the way to Mexico.
You could then go up to Palenque and San Crisobel de las Casas around the Yucatan--Veracruz, Merida (lots of ruins around there), Chichen Itza and back to Cancun.
So many delicious possibilities--enjoy!