some experiences
well, althought the news can be awful about Guatemala, but with common sense you can travel perfectly in this country!
just take shuttlebusses...

Semuc: a real "you have to go" place
I prefer to stay at El Retiro in Languin..... the most beautifull hostel i stayed, with good food!!!!!!
the tour to the pools and cave is really great!
If you just want to see the pools, its easy to take a pick up from Languin (so i should say: don't stay in Semuc, but choose El Retiro in languin).

Lago Atitlan:
San Marcos is sooooooo great, not a party place, but just a really nice place!
If you want to do hiking there, you need a guide (with the real equipment), i can strongly advise to go with Mario Gomez. He lives near the Iglesia (church), asked around there for him or call him: 5540 5819 (please if you phone, you have to speak a little spanish).
to walk from San Marcos to Santa Cruz will take 4 or 4 1/2 hours and will cost 150 quetzales. He is great and with him it's a safe and very pleasant walk!!!!

Enjoy!!!!!!!!!!