Santa Monica -- so many choices -- There is a great hands on Science Museum Downtown, There is another in Pasadena Kids Place (or something like that), Huntington Gardens, Walk the Boardwalk, The Skirball has the Noah's Ark Room, The Getty Center Gardens are great fun to wonder through. Go to the library and get a Walking Tours of Los Angeles book, and take a walk around the world visiting Chinatown, Olvera Street, The Farmers Market, Little Italy, Little Ethiopia and more.
Think of yourselves as tourists in your home state and plan lots of places to go and keep having days of fun things to do.
Take snacks, maybe a picnic, a camera and a willing spirit and you will make great memories together, -- wow, have a special day!