The laptop won't power large speakers on it's own, so you will be spending money on something...what that something is all depends on the size of the area you are trying to cover, your finances and eventual goals.
Simplest is to just get a cheap set of powered speakers with a subwoofer made for computers and a long extension for the standard 1/8" stereo jack. You have to protect it from the weather and not going to project real far though.
Examples:
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LoginIf you are planning to run a show, you could just use an FM transmitter and an all weather radio. Here is the setup I use..whether on foot or by car, visitors can still hear the show. Plus I can use the radio for the rest of the year at the beach, tailgating, whatever.
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LoginGood luck and have fun with it,
Mike E.