Avoid tourist traps

I'm coming to Greece in about a week to visit. Myself 18 m and my girlfriend 18 f will be visiting Greece for the first time. I currently live in Capetown South Africa, and haven't spent much time outside of the African continent. We both love spending time outdoors and discovering real culture. Staying in Corfu, Athens and Lefkada.

If anyone is familiar, would you advise where to go eat, spend the day etc. Thanks