I would suggest the typical all-American hamburgers, hot dogs, chips, dip, vegetable plates and ranch. Also, this may seem odd but a friend of mine always serves taquitos (beef or chicken rolled into a tortilla in a stick form) at her pool parties. You can even get these frozen at Costco or other grocery stores. I definitely recommend them they are absolutely delicious!=]
As for the activities, definitely have cool, up-to-date music, karaoke, volleyball, and any other fun things that your friends usually do. Make sure you think of some activities for the non-sport playing guests, I can sympathize with them =)
And the invitations, you can never go wrong with a simple statement that covers the point of the party such as:
Come join us for an unforgettable pool party to celebrate the end of the summer!
But most of all, relax! You will have a great time. Enjoy!