The City of San Luis Obispo has four community gardens sites with over 100 plots available for City residents to lease. Garden plots are available to City residents on a first-come, first-served basis. The annual fee is a $25 base rate plus a $.03 assessment charge per square foot. Call 781-7300 to be added to the Community Gardens waitlist.
The Rotary Garden contains 40 plots. It is located at the west end of Meadow Park with the nearest access on South Street. You can take a virtual tour of the garden here: bit.ly/RotaryCommunityGarden
You're looking for a micro with a log only. Maybe room for a couple very small sharables. There is no need to go into anyone's garden plot to find this cache. Park on the opposite end of the park and enjoy a brief little walk across to the garden.
Beware muggles of the walking, biking, driving, and gardening varities. Probably best to approach early in the morning or after dark. BYOP.