The Discovery museum in Orange County is a fantastic place to get a family membership to keep the kids entertained. They have new exhibits every 2-3 months and definitely worth considering the membership. As with most kid-crazy place you want to get there early in the morning to avoid parking and crowd headaches.
Currently they have a Lego Travel Adventures exhibit that runs through May 2, and if your kids need to get their Lego fix, or secretly you need a Lego fix, this is a average size exhibit that will eat up about 30-45 min of your visit. The theme is traveling and most of the displays are of popular transportation methods set across different cities throughout America.
There are a couple of interactive areas where you can build cars and race them down a track similar to what is found outside many Lego stores. These however, use Duplo blocks so they are bigger and more friendly to younger audience. Most of the other Lego displays are mainly used for photo opportunities.
The rest of the time can be spent exploring the myriad of learning and discovery exhibits and can take hours of your time. My kids really enjoy the upstairs grocery shopping experience and the recycling contest. The Dino discovery outdoors is decent but you will need to pay an additional fee. We go every couple of months so I like to spend about 2-3 hours there and then head home with plenty of time left in the day for myself or the fam.
Tip: Construction is still in progress as they are expanding and parking lot is limited. Save yourself some time and money and just park at the Santa Ana mall across the street and walk over.