Go to the beach... Huntington is nice, there is a nice pier with a restaurant at the end. Fun for the kiddos to watch people surf or fishing.
Bring a small kite (nylon type that folds up small with no sticks) to fly on the beach..less temptation to get in the ocean if they are occupied.
There is a science museum in Santa Anna, good for kids. My teenagers were bored by it.. but it would be perfect for younger children. They have a dinosaur outside that you can climb inside and see how it's digestion works. Lots of clue finding with a special pointer that you rent.. I'm not doing a good job describing it.

If my boys were younger they would have loved it.
http://www.discoverycube.org/
With a longer time span at DL, you will have the opportunity to watch the entertainment offerings. Taking time for Meet and Greets , stop and watch strolling musicians, etc.
Bring a sack lunch and hang out for awhile.. just people watching.
We've had some magical moments doing a "slow" day, you notice the little things more. No need to be frustrated with crowds or "we need to be at X by X time or the day is ruined" thinking, you have time.
Make sure to explore the grounds of the DL Hotel. Chck to see what time they feed the carp, the kiddos will LOVE helping out! Explore the waterfall paths. Play with the remote control boats. Walk inside from Goofy's to the main lobby and you will find some nice paintings, photos, and display cases.
On early cold mornings, take a seat at the outdoor fireplace of the Grand Californian Hotel with a cup of hot cocoa.
Drat, I know I shouldn't have looked on the DL board.. Now I miss it and want to go back boo hooo hoooo.
Many of my friends said I would tire of DL, we moved to the area last year.. visited many weekends and days off of school.
There is much to do and see .. much to sit back and watch.....
I never did tire of it.
Hope you have a wonderful time!