What is your time frame?
We made this trip from Central Mexico to Panama and back in 9 weeks from January until March this year and it wouldn´t have been boring if it had been half a year! We drove over 14 tsd km with some 1-3 days breaks on nice and interesting places
The Panamericana is not really what you need, when you want to have fun.
This was our tour:
Combining the places where we´ve been, for a one-way south-east, I´d travel:
Town / Region / estimated days for traveling and stay
Arriving? 2-3
Oaxaca (Town and Monte Alban) 2
Puerto Escondido (vía road 131) 2
Chiapa de Corzo (Cañon Sumidero) 2
San Cristobal de las Casas (on toll-free road 190) 1
Palenque (Agua Azul and other waterfalls on the way,
Pyramids…) 2
Chetumal 2
Crossing Belize (on the beach, different places, we
stayed 2 nights in Hopkins) 2
Tikal N.P 2
Coban (eat a steak at "al carbon d´ fioara") 1
Panajachel (Lago Atitlan, 1 day for a tour around the lake) 2
Antigua 1
If you need parts or help, Guatemala City is the place "completest" and cheapest place in Central America,
Rio Dulce (N.P., boat ride to Livingstone and back) 2
Crossing to Honduras in direction San Pedro Sula
(for us the easiest Hondura border)
Lago de Yojoa, Honduras 1
Leon, Nicaragua 2
Granada 1
San Juan del Sur, Nicaragua 2
Nuevo Arenal (Laguna de Arenal, Volcano), Costa Rica 1
Puerto Viejo, Costa Rica 3
through Bocas de Toro, Boquete 2
Panama City 4
Already with this tight schedule, it`s a 6 weeks trip, each day more will improve it definitely.
A trip report in german with pictures, you find in the gs-forum.eu.
We live in Leon, Gto and Puerto Vallarta, Jal, if we can help with more details, let us know.
Saludos
Uli