Get all the necessary contact, medical and legal information for every child who is participating well before the program starts. Make sure that everyone working in the summer program is aware of where the information is kept and of any medical, dietary or personal issues related to the children long before the kids arrive. Make sure you will always have the required first aid provision in terms of kit and practitioners. Once you have a program for the activities you will be doing, get a blanket parental consent form sent out to the parents and don't accept kids in the program until you have the form back with a signature. Usually these programs go without a hitch, but being prepared is never a bad thing.
Take account of how many children and how many helpers you have when planning activities. Set a sensible cap on the size of the program. There are usually legal limits regarding the ratio of children to workers, but one to eight is generally a good ratio.
Set up the day and the week into a regular pattern so that the kids get used to the structure. Organize the day so that it starts with a group activity, like parachute games, and has further group activities -- lunch, story time, outings, sports etc. -- interspersed with about an hour of free time in which kids can entertain themselves. This gives a steady flow of input to the kids, keeping boredom at bay while still offering a lot of free time to explore their own imaginations.
Organize the weeks around themes and base a weekly outing on each theme. For example, you could have an animal theme, starting the week with a trip to the zoo and then having animal stories, animal face painting and animal arts and crafts sessions. You could then do a theme on food, visiting a local vegetable grower or farm then have the children plant some of their own vegetables and do some paintings of people and faces made up of fruit and vegetables.
Allow the kids to choose which activities to do one day per week. Let Friday or the last day of the week that the program runs is a good day to be planned by the kids. Let them choose the activities they want to do, but try and get them to keep loosely to theme of the week. Help them develop any ideas that they have and make sure everyone's voice gets heard.