San Luis Obispo is well known for it's Thursday evening Farmers' Market in Downtown. I suggest you go if you are in town on Thursday, but many travelers are here only for the weekends. If you are staying in SLO I suggest you hit the Saturday Morning market at the Madonna Plaza. This market has a great mix of local fruits and vegetables as well as prepared foods, breads, local cheeses, salami, meets and more.
Starting at 8 am in the World Market and Embassy Suites parking lot it is perfect to go there early, pick up all the supplies you need for a picnic and go enjoy the beautiful Central Coast. SLO Wine Country Association sells wine 2 Saturdays per month. If you need other supplies, there is a grocery store on the opposite side of the shopping plaza or World Market.
http://www.slocountyfarmers.org