Of course: comfy walking shoes and a warm (lightweight) topper. You are correct that Monterey weather is often similar to SF weather: warm sun, cold winds, sometimes cold damp. You are likely to be both too cold and too warm, maybe on the same block. The vibe is casual for sure and also touristy for sure!
Cashmere might work (but may not hold up to breezes); fleece is perfect (but can be frumpy); leather moto may also be perfect (if you can carry it without becoming annoyed). A sweater plus a thin (stuff-able) wind breaker might cut it. Of course it depends on what you do. For example, walking to the Monterey Bay Aquarium may be chilly, but inside is warm. Thus my emphasis on warm but easy-to-carry.
The beaches are beautiful and photogenic. Fleece is your best lightweight cozy option there.
Have fun!
