The Getty Villa
A place I recommend to those visiting Los Angeles is The Getty Villa. It is not an all day type event, but definitely somewhere you should drop by if you are already on the West coast visiting the nearby beaches. It is a very beautiful place, assuming the weather is nice.
If you are planning to come here, you do have to reserve a time slot for you and your guests. It is very simple and free. I also recommend packing a nice snack or lunch - there are many places to sit and eat, and just take in the scenery. The beach is directly across the street, down the hill, so you can imagine the amazing views.
Even if you are not a history buff and have no interest in Roman architecture, this place is still nice to stroll around, take pictures, and have a mini picnic.
Make sure you check the weather!