Scott,
Try some of the inexpensive SPD's mentioned in the previous posts. The ones with SPD on one side and a standard quill/platform type on the other. This will allow you to do what you percieve you want to do. Jump on the bike with any shoe/sandal for a short trip or put on a cycling shoe for a day ride. Yes you can ride with some of the others but they are more expensive and do not provide the support and grip that a platform/quill pedal will when used with assorted shoes including the feared flip flop. You are just starting out, you may well graduate to other pedals in the future but for you current stated needs this IMO is the way to go.