I'm travelling round Europe very soon. What are some pro-tips for 'surviving'?

So, me and a couple of friends are going round Greece for two months in 3 days.

We're all 18, so this is new to us, (we have been away by ourselves numerous times, but only for a week or so), so what are some things we should bring, and things we should remember? Either to just keep ourselves entertained, or incase of emergency type preparations. Anything would help...