I highly recommend starting a Guatemala trip Antigua instead of Guatemala City. You can take a shuttle there from the airport and it takes about an hour (traffic depending). From there, Make sure you visit Lake Atitlan. The nature is amazing and there's so many different communities around the lake. Just make sure you go swimming there at least once. Just try to support the local places instead of those run by plundering ex-pats. Two regrets I have though are not making it to Tikal. Tikal is said to be some of the most amazing Mayan ruins in the world and a friend of mine who went said the experience was absolutely mind blowing.

Make sure to go hiking and exploring the volcanoes. They're all over the country and they're an incredible sight to see. One of the best activities for this would be to hike up the mountain Aguatenango and spending the night at the base camp. If you do this, you get the opportunity to see the nearby volcano erupting throughout the night and at dawn, which makes for an absolutely unreal experience. Just make sure you dress warm because it often drops below freezing during the night.

Although the country has a high crime rate and there are many safety warnings, I personally had no problems and saw no incidents while I was there. There were many armed guards in public places though, but I didn't feel unsafe at any time. From what I've heard, most of the crime is gang related and located in Guatemala City and the eastern part of the country. I spent my stay in the west. Generally speaking, just use good judgement and you should be fine.

Overall Impressions

So many volcanoes (and they're awesome)
Lake Atitlan looks like a setting from a fantasy novel
Local food is delicious
Antigua is a really cool historic city
Matiox Hostel in Antigua is now one of my all-time favorites
People are generally very friendly
Tuc-tucs are fin to ride but can also be a bit nerve-racking (such as if your driver is 10 years old)
Transportation is a nightmare
Everything is so colorful
Many Mayan people were traditional clothing
Drinking the coffee here felt like a mystical religious experience
For January, the weather was amazing