Celebrating Lag Baomer. Lighting bonfires and going to Meron.
Going to Meron is definitely a custom (the whole celebration of Lag BaOmer is a custom), and according to this article, over 500,000 people were expected to make the pilgrimage in 2009.
According to this, as of Iyar 2011 there were 5,837,000 Jews living in Israel, so that's a pretty large percentage.
As for lighting bonfires, I don't have any numbers, but my understanding is that it is done all over Israel.