If you're interesting in some adventure, I'd highly recommend trying paragliding. It involves running off the side of a mountain with a modified parachute strapped to your back, but once you get past that, it's a great and peaceful experience. You certainly can't beat the views as your glide over the hillsides and the lake itself. Arrangements can be made in Panajachel and the actual jump will take place on a hill above the village of Santa Catarina. The price is around 450 Q. Check out my Panajachel page for more details.
I didn't do it, but I head there is at least one scuba diving outfit in Lago Atitlan. The water is crystal clear on many spots so I bet it will make for a nice dive or two. I hadn't learn how to dive at this stage of my trip so I gave it a miss. I'll return there someday though to give it a try :).
I had a great kayaking tour with Los Elementos Adventure Center this month. We started from their base camp in Santa Cruz la Laguna, and paddled out to the middle of the lake to watch the sun rise. It was beautiful and serene. The perfect way to start the day. Afterwards, we paddled west towards San Marcos. Along the way we passed steep cliffs and saw some lake heron. After a breakfast in San Marcos, we paddled back for our return. We stopped between the villages of Tzununa and El Jaibalito to jump off some rock cliffs. What fun and excitement for one morning.
Later in the day we had a hike up the Pumpatin Canyon to find the Hidden Waterfalls and have a refreshing swim.
The staff at Los Elementos Adventure Center were helpful and knowledgable about the Lake Atitlan area. You can choose between english and spanish speaking guides.
Equipment: Everything is provided for the days fun except sunsreen and your camera.