When you arrive to the airport in Guatemala City, go outside and up on the upper floor where the ticket counters are located. Around the corner from the Iberia ticket counter is a Banrural that will exchange your money for you at a much better rate than the exchange centers you'll pass by before immigration and customs. Also, there are 2 ATM machines behind the other money exchange center located at the ticket counters. Those will not be tampered with, I assure you, due to the security inside the airport.
Furthermore, you can exchange money at most hotels and also at banks in Antigua as well. It's not a hassle to do so, either. You will need to simply show your passport at the banks and tell them "yo necesito cambiar dolares por quetzales" and hand them the USD. They'll take care of you. No need to have an account there or anything else. They might charge you a small fee to exchange the money, that's all though.