Weather should be 'pleasant', but probably a bit chilly for sun bathing. My guess would be high 60's F (20C?). The Pacific Ocean is always chilly!
Santa Monica is a very nice sandy beach with a pier, restaurants, shops etc close by. You can hire a bicycle (or blades) from the pier and cycle along miles of cycle track between Malibu and somewhere much further South. Possibly Long Beach.
TG.