What are some good games for teenagers to play at a park?

I teach a teenager’s class in church, and the girls and boys (ages 14–15) are going to get together at a park with a lake, volleyball court, field, playground, etc. We were looking for good games that both genders would enjoy, preferably not costing anything (we don’t exactly have the biggest budget!) Also, we are planning on having a picnic. Ideas for anything would be greatly appreciated!