This map shows the local beaches that are good for kitesurfing... (15 min drive from the city)

If the wind is on, there will almost certainly be people out at Largs Bay, West Lakes and Henley Beach. It'll be a mixture of flat sandy channels, small chop to shoulder-high waves depending on the tide.
Further south... 30 to 60 min drive... Moana and Goolwa get waves. Less people, can drive and park 4WD on the beach.