That's accomplishing a lot in one day; as mentioned, the beach is out of the way, and driving through Los Angeles (the city and its adjacent suburbs) plus the beach during the summer will always equate into a slow commute.
My personal suggestion: use the Metrolink. The station is across the street from the airport, and connected by a designated crosswalk with traffic control. Take the train into Union Station, then take the subway into Hollywood. Take the subway back to Union Station, then take the Metrolink to Anaheim. You get free ART access with your Metrolink ticket, and can take it to your hotel. On another day, ride the Metrolink south into Oceanside. It takes a couple hours, and the last hour the train rides along the water. From the Oceanside station, it's a short walk to the beach/pier. Alternatively, you can get off at San Clemente, which yields a shorter (less scenic) ride, but the station by the pier has limited scheduling. The fares are fairly reasonable, and some include transfers. On weekends, you get unlimited access for $10. Kids 5 & under ride free ... but they're pretty liberal about this, so if you have young kids, they'll pass.
Much less stressful than driving (try finding street parking in Hollywood... be aware of restrictions! Monday afternoon is OK, but you'll be towed in the evening ... Tuesday afternoon is a no go either but the evening is OK ... and is that red or purple? I ca't tell), and likely cheaper than a one way rental.