There are many PA system available for use outside on an ad hoc basis. If you wish to take the equipment inside when finished. For an outdoor stage you should consult with the Dolby Professional division. They have speakers that can be mounted and left outdoors, and still sound better than a string and tin can like a lot of outdoor horns (think train station announcements.) In my outdoor venues, I usually mount the mixer inside, and allow the operator to peer out a window to the guest area. Where not possible I house the equipment in a rain tight box with air con running 24/7 to prevent corrosion from spoiling the equipment. Generally a lid is lifted or removed to find the show controls inside. Usually these are integrated with theatrical lighting controls as well. this allows a "one man band" to control the whole show.
You need to consider how many people and the crowd noise level to determine how many speakers and what amplifier power you need. A low density application, like dining, can be accomplished with 50 watts per channel. A dance situation with rock music will require a 400 watts per channel system to create the requisite din required by a well attended rock concert on 10,000 square feet.
