Comments : The basics are there but it is not a place to longer...
What to do ? On arrival : Queue at the immigration / Get a nice 90 days visa stamped on your passport / Wait for your luggage / Change moneys (a few places) / Get tips & info at the small but helpful tourist kiosk (6am-9pm) / Say 'no gracias' to taxi touts / Say your first few words of Spanish (English is still OK, though)
What you may not like ? The late arrival time of most flights / Queuing or waiting too long / The limited documents available at the tourist kiosk / The US$25 airport tax : you could spend 3 days in the country with that !
How long ? Half an hour or so. It is a fairly relaxed airport but there aren't much to do.
Where to stay ? If you arrive in the evening (and most flights unfortunately do), you should seriously consider staying in the area rather than heading for unsafe Guatemala City in search of an hotel. Dos Luna GH (21 C 10-92, Zona 13, Tel : 3345264, firstname.lastname@example.org) offers correct rooms for US$10, incl. pick-up from the airport. It is recommended to reserve in advance.
Banks ? Several places to change money & TC at good rates. The Banco del Quetzal agency exchanges all European currencies, US and Canadian dollars and is open from 6am to 9pm.
How to leave ? Most people nowadays do not bother with Guatemala City and head directly for Antigua. Three options from the airport.
The first two should be ok in the evening but note that walking by night with
your bags in Antigua is not recommended.
The trip to Guatemala City (Zona 1) : S / L&R / Q1 / 20mn
Onward to Antigua