Are you staying at the Embassy Suites in El Segundo, near the LAX Airport? You might try visiting Manhattan & Redondo Beaches. The Redondo Beach Pier isn't too far away, and there are a lot of nice restaurants to visit that first night.
If you're lucky you'll get here when we have the Santa Ana's, and then we'll have temps in the high 80s. However, it might be more likely that temps will be in the 70s.
If you'd like, you can rent bikes, and ride them up to the Santa Monica Pier, or towards Redondo Beach Pier on the "strand." Going to Sta. Monica Pier, you'll ride through Muscle Beach & Venice Beach.
You can also drive from the hotel, and make your way to Orange County, via a more scenic route, going around the Palos Verdes Peninsula, and stopping at the Point Vicente lighthouse. There is a spot there for whale watching and you can go in a little visitors center there.
http://www.palosverdes.com/pvlight/
You can tell your kids that just to the left of the lighthouse is an area where Marineland used to be and where they filmed part of Pirates of the Caribbean. The Black Pearl actually sailed that area.
Continue around Palos Verdes, and you'll come to another overlook where you can see San Pedro, Terminal Island (prison), the LA Harbor, and the Queen Mary in Long Beach. Go down the hill to the Cabrillo Beach area (where they filmed part of Face Off), and visit the Cabrillo Beach Museum. There are also tidepools in this area.
http://www.sanpedrochamber.com/champint/cbrobch.htm
Continue past Ports O' Call, and the cruise port where the Disney Magic will be this summer, and cross the Vincent Thomas Bridge to Long Beach.
In Long Beach you have the Long Beach Aquarium of the Pacific
http://www.aquariumofpacific.org/ that you can visit.
If the above will take too long, you can take the freeway to the 110 Harbor Fwy, go south & then go over the Vincent Thomas Bridge to Long Beach. You can continue along the coast and visit Long Beach, Seal Beach, or continue to Huntington Beach & Newport Beach. There are also bike rentals at both Huntington & Newport Beach.
If you rent bikes at Newport Beach, you can take the ferry to Balboa Island & ride over there (the ferry is about $1/person - may be a bit more now.