Choose a beach that is not populated a lot. It should allow friends to stay together and do silly stuff so that they can relive the moments from the past but it can only happen when they are not bothered of other people judging them. It can be a beach that you guys went to during your teens etc or anything that can be associated with a special memory.
Book a hall for the late night party. An indoor location is essential after a day at the beach so make sure that you can get your hands to some location that provides roof. Call different hotels to know the rate and then choose the one that best fits your budget.
Send invitations to the friends via text message of facebook event. Tell them the exact time and location of the reunion and briefly discuss the activities that can be incorporated in the plan. Include the information about the hotel where the hall is been booked.
Hire a DJ to keep the night alive. Prepare some slideshows as well that will contain pictures or videos from the past; some funny but memorable moments.