Isola San Giulio is little more than a church and a single path around the island less than 200m in length. The path is called the "Path of Silence" and has little sayings every 25 m about the virtues of silence. We think it was just a ploy by the residents to try to curb the noise created by the scores of visitors who taxi over each day. It didn't work. Most of the (primarily Italian) tourists we encountered were content to jabber with one another or on their cell phones the entire circuit.
The church is small, but very picturesque and interesting. Go early and do the circuit first while most go into the church. You may have the path in silence after all and if you get to the church before the next water taxi lands, you can have the church to yourself too!
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