Don't skimp on long distance bus rides. Chicken buses are fun for a short time, but most will not want to spend a long time on them.
The route via Poptun is served by two "luxury" bus companies, [http://tikalmayanworld.com/services/horarios.htm/ Linea Dorada]and [http://www.adnautobusesdelnorte.com/eng/quienes.htm/ Autobuses del Norte]. Note that Linea Dorada's website claims are not borne out by facts and ADN advertises having "First Class Bonus". The advantage of this route is that you can go overnight and not become jaded by slash and burn agriculture for hours.
The route via Coban takes about 12 hours on buses, as opposed to the 10 or so via the other route, but it is more scenic. Plus, though you have to change buses in Coban, that is a destination in its own right. You would start out from Guate on [http://www.autopullmanmonjablanca.com/home.htm/ Monja Blanca] and switch to a minivan in Coban.
Two good beers in Guatemala are Gallo, and Moza (which is a sweet-extrabody beer).
In most of the restaurants there are also fruit, smoothy type drinks called Liquiados, that come in a variety of fruits, and are made from fresh fruits.
Always stay on the beaten path, and stay with a group. Don't wander off, stay alert, and try your best to blend in and not draw too much attention to yourself.