My own favorite(s)
My own favorite is Bay to Breakers, the annual running race cum crazy party across San Francisco. There are plenty of naked people there, but if you're not going with a group of nakeds (many do), then among all of the thousands of participants, you're likely to go for blocks without seeing another naked person. It's fun to run into the occasional naked person or group. And it's fun to walk along city streets through crowds of (more or less) dressed people, and be the only one naked.
I also did Burning Man in 2009. I effectively lived naked for a week in the high desert of northern Nevada. (I put clothes on only for bed, because it was SO cold.)
But in a way, Bay to Breakers is more fun--at least in this respect. Living, going about, strolling naked at Burning Man--even when you're the only naked person about--feels very natural and ordinary. At Bay to Breakers, it's still a little naughty. People will run up to me, just to get a picture taken with the "naked guy."