Guatemala is a great place to visit but the issue of money matters does make it a challenge. Here are a few thoughts.
First, cash is king in Guatemala. Most businesses do not accept credit cards. Therefore you will need some cash. Most major banks can provide you with quetzales. There exchange rate is bad but you will have cash before you leave for Guatemala. It normally takes a week to get the quetzales unless you live in a major city.
Second, ATM's can be risky in Guatemala but they are very handy and convenient. If you use ATM machine, try to use machines inside banks or major hotels. If you plan to use an ATM, talk to your bank before you leave. Let them know the dates of your travel. Ask that any foreign transactions after your planned trip be denied. Ask about the limits of cash withdraws. You may want to limit the daily withdrawal levels to no more than $200 or $300 per day. Many banks set the default at $1,000 per day. Also, if possible set up an account that is not connected to your savings account or other accounts. This will limit your exposure. If you have trusted family or friends not traveling with you, you may want to lower your balance in your account and have them transfer funds/deposit funds as you need additional funds.
Third, travelers' checks are safer but a pain to cash. I like Citibank traveler's checks because there is a Citibank branch in Antigua. You will need your passport to cash them. Plan to spent at least an hour because of the wait in line and the paperwork involved. Saturday seems to have the longest line.
Fourth, Guatemalan prefer USA currency to Euros or Canadian dollars. They also prefer new USA dollars. Some place will not take torn, ripped or even nicked dollars. They can be fussy. If you know this up front, you can be prepared.
Fifth, the airport have several currency exchange places. There is usually a long line at the first exchange as you leave the airplane. There is another place at the place of the stairs just beyond the first place. I have never seen a line at the second exchange. There is a third exchange past passport control before you get to the luggage carrousel.
Sixth, I try to avoid denominations larger than Q200. For many businesses even making change for Q100 can be a challenge. If you know you have some larger purchases such as hotel stays or school tuition, larger bills are okay. But, for walking around money, the smaller bills are handy.
Seventh, banks will require passports to exchange currency into quetzales.