If your teen is athletic, stress all of the nature hikes, rappelling, snorkeling, etc. If your teen is interested in the environment, look for a program on a nature reserve, etc. Present Israel as the only democracy in the Middle East, a miracle of a nation which in 50 years has created a successful, although imperfect, modern society and economy. Talk about an Israel trip as a formative Jewish experience, as important as a bar or bat mitzvah. Find someone who has been there recently to talk to your child.
Nothing works better than hearing what a good time a friend had.
And, have your child look at Israel-related web sites on the Internet.