When I can't think of something to eat and I want something good and cheap, I head on out to the main drag (the street with tons of shops and restaurants). Off to the side of the road a ways, you will find a little charcoal grill and on it is corn on the cob. I've never had grilled corn on the cob before and I have to say I am a huge fan. They cover it in lime and salt and it is delicious and they are only Q5 each. What a deal!
There are all kinds of delicious foods around Guatemala and I'm having a blast trying them all out. :)