Volunteer on the Lake
I think the best way to get to know the community is to connect with a
trusted Non-Profit serving the community around the lake. I worked for a non-profit for six months called Guatemala Housing Alliance, and it was one of the most enriching experiences of my life. It quickly and effectively gets your past the facade of nice hotels and shops into parts of the community where you can experience what the Mayan Culture and people are really like. Also, volunteering works well in tandem with Spanish language learners.
Stay active, keep a Schedule, and learn Spanish!
With all the new people passing through and partying going on it's easy to get lost in that whole scene. You'll always see at least one guy at the bar ordering a coke because "I'm in detox, I've been drinking every day for months" then later claiming he was never like that at home. Being so disconnected from home and not having any responsibilities on the lake is great but can also makes it difficult to maintain a healthy lifestyle. Remember, you're living in a small community of locals who have a long history on the lake. Don't be one of those gringos they resent.
I recommend learning Spanish, keeping a schedule, and staying active. Being able to engage in conversation with locals is vital to connecting and being part of their community. People are early risers on the lake, and you can get an excellent cup of coffee at 6 a.m.. Volunteering or Spanish classes is good for keeping the schedule as well. As for affordable activities, I love going horseback riding, hikes around the area, and playing futbol in the stadiums.
Mingle with the Ex Pats
You think the Mayan Culture is foreign, wait until you meet some of the Expats living on the lake : ). Good places to meet these people are in local events such as Pub quizzes, poker games, and the infamous BBQ. They too will be very helpful with helping you navigate your way around the lake.
Start your Own Project
This is a little bit trickier. But as you get a feeling for your community and building relationships it might be in you to start something of you own with the locals. One of the things they are in need of is good jobs. I did and it's one of the best decisions I ever made. www.atitlanleather.com is an online artisan leather goods shop. We specialize in money belts and other travel accessories so make sure you take a look before your next trip!