Okay, I've done all my homework...thanks for all of the advice.
Now I just need one question answered.
We have 18 days, Here is what we plan to cover all by bus/shuttle:
Antigua first-Pacaya volcano overnight (only thing booked), Copan Ruins, Flores/Tikal, Rio Dulce/Livingston, and then chill at Lake Atitlan for the last few days of the trip.
I chose this order because I want to break up the long trip to Flores by visiting Copan on the way there and Rio Dulce on the way back...does this make sense or am I doing more backtracking than I should? Should I just do Rio Dulce after Copan on the way up to Flores and then suffer the long ride back to Pana in one shot?
Thanks in advance for any additional advice.