Atitlan is approximately 3.5 hours away from Guatemala City if you're lucky. We got stuck on a road 30 minutes out of the city for over two hours because of an accident. What should have been a 2.5 hour trip (because we took the shorter route) ended up being a 5+ hour trip. I recommend taking the longer route if you don't have a sufficient car because the roads can be trecherous due to the steep hills and rocky roads. Lake Atitlan is a large body of water in volcanic crater. At any given time of the day, depending on where you are, you can see any or all of the three vocanoes surrounding it.
Depending on where you stay, and depending on the kind of traveler you are (camper, hotel, airbnb), you will be met with a lot of different conditions. Have no expectations. Lake Atitlan is beautiful and there are many sides of the lake you can stay on. I stayed in Panajachel after canceling an Airbnb that had reviews of gigantic spiders and scorpions in the kitchen and shower (No thank you!).
Panajachel can connect you to many other parts of the lake by boat if you wish to venture out. There are many activities and lots to do.