A young man from Costa Rica said Panajachel was too cold. If you are wanting a very hot, tropical climate when you come, you may be disappointed. Be sure to throw in a sweater to go with your shorts and flip flops.
Guatemala has two seasons: wet and dry. The dry season--December to May-- is sunny and warm during the day but can get cool in the evenings. The Lake Atitlan region is at an elevation of up to 5000 ft, and the higher up areas can get quite cool. Also, there's no central heating in hotels, homes and restaurants, nor air conditioning either. I suggest bringing something warm for sleeping, in case the bedding at your hotel isn't sufficient.
During the day, it can get hot as you walk around town, and that's when layering works best. A jacket or sweater for cool mornings can be shed during the day and put in your backpack, ready for sunset. I like to put in my flip flops too, in case my feet get too hot in shoes.
If your eyes are sensitive to the sun, bring a couple pairs of sunglasses. I'm never without mine. Buying them locally can be pricey, as it's one of those "the tourists will pay for them" items.
In the rainy season-- May through November-- the time to go out is in the mornings when the sun is shining. Take an umbrella in case you get caught in the rain which nearly always starts by 1 or 2 pm.
That's when I like to be home next to my fireplace, with plenty of dry wood nearby.