If you only visit one fun park in the USA, make it Disneyland.
Located in Anaheim California, a short distance from Los Angeles,  the first Disneyland Park ever built,  is so much fun for big kids just as much as for the little kids. I think it is the one place in the world where you can forget about all the pressures of everyday life for a few hours and lose yourself in the excitement and wonderment of such an amazing place.

Some of the highlights are:
Pirates of the Caribbean, Space Mountain, Haunted House, Matterhorn Bobsleds, Indiana Jones Adventure or Dumbo the Flying Elephant,  just to name a few.

The queue’s at the park can be very long and you want to make the most of your time there and see as much as possible, so it is handy to know about the Disney FASTPASS Service.
Select attractions have the Disney FASTPASS Service, a complimentary program that allows you to save your place in the queue, while still enjoying the rest of the park. This is subject to availability.

There is also a facility called Rider Swap where one parent can wait with a child who is not able to go on a ride and can swap with other parent to use the ride on their return.

You will need to get to the park early, wear comfortable shoes as it is a long day in fact I would allow at least 2 full days to enjoy the park.  

There are plenty of hotels adjacent to the park for the weary tourist to save traveling back to Los Angeles at the end of the day.
Destination Diva has all the information to make your visit a memorable one and advice on the best places to stay to suit your budget.

http://disneyland.disney.go.com/attractions/