So sadly, I don't appear to have to much about my trip. However, I crossed from Belize into Guatemala and went straight to Flores in the North. From there we camped a night at the Mayan ruins at Tikal. I distinctly remember the amazing noise of the monkeys in the Jungle at dawn. The ruins themselves, whilst quite touristy are absolutely amazing, a real must see. From there we travelled down to Antigua, a gorgeous place, though I have no information on the town I'm afraid or Lake Atilan, where you are heading.
Guatemala itself is typical of Mid/South America. Lovely people and stunning country, but interspersed with lots of poverty. Safety wise, as long as you take sensible precautions, I found it to be fine. I don't know if you've travelled much in that part of the world? I've travelled quite extensively in South America and found Guatemala to be very similar. One of the big things to remember is that folk are generally really eager to please, so you need to be very specific when you ask questions. Something that got be into a bit of bother a couple of times! In common with the rest of the continent, they speak a softer version of Spanish as well which I found much easier to pronounce. I carried a Latin American Spanish phrasebook which was invaluable when my Spanish let me down.
Sorry I can't be more helpful, but I hope you have a lovely time, I'd really love to go back. I'm sure you'll have a fantastic time and you've reminded me that I really need to digitise my photos so I can preserve them.