I personally think Lake Atitlan is one of the most gorgeous lakes in the world. The image of volcanoes towering over the rippling water, the fisherman waking early to set off in their wooden boats and the sun setting between the volcanic peaks are experiences not to be missed (and why would you since the lake is only 3 hours from Antigua). Lake Atitlan also offers the explorer many thrilling activities from hiking to paragliding, or cultural experiences like boat tours and visiting local deities.
The lake is surrounded by many villages in which Mayan culture is still the predominant custom. Head to these communities to see the traditional dress and view colorfully authentic textiles and fabrics produced by these towns. Santiago Atitlan is most notably known for housing Maximon, an idol created from a fusion of traditional Mayan deities, Catholic saints, and conquistador legends. Panajachel (Pana) is the usual tourist town boasting of restaurants, shopping, and an abundance of hotels. Pana is the most common destination for visitors heading to the lake. While Pana is worth checking out, I think it detracts from both the serenity of the lake, as well as the appreciation for the traditional lifestyle of those who live there. If staying overnight, look at places that are a little farther removed from the tourist scene. My vacations to the lake always involve a stay at La Casa del Mundo in Jaibalito. Trips also include hiking and kayaking. One of my favorite hikes is the San Pedro Volcano which gives way to incredible views from the top.
Even if you are in Guatemala for a short time, head to Lake Atitlan (if only for a day trip). Whether it's a weekend or 1 day, you will be awestruck at the serenity and beauty of this incredible lake.