Thanks for the suggestion @laurenfinley. Feel free to contribute your own tips.
  1. Bring Imodium .
    Even if you avoid drinking the water, there is a good chance you will be spending extra time in the restroom.
  2. Spend a couple days at the lake.
    Lake Atitlan is beautiful, and definitely worth a visit. But it isn't enough to to do a day trip. Take a couple days to explore the lake and the surrounding villages.
  3. Splurge on a hotel with air conditioning.
    If you are staying in Flores (or near Tikal) during summer, stay at a place with air conditioning. It gets very hot and very humid. If you want any energy to see the sights, you will need respite from the heat.
  4. Haggle haggle haggle.
    You will probably want to return home with some merch. Haggling is popular and expected in Guatemala. Only suckers pay sticker price.
  5. Avoid the tour guides around Lake Atitlan.
    They just try to get you to buy their buddies' stuff.