I realize that Universal is in Hollywood, not Anaheim, but it is a common side trip while visiting Disneyland.
Universal can easily be done in a day - it closes at 6:00 pm & the stores in the shopping area are open later so save your shopping for after the park closes. When you arrive at Universal, head straight for the lower level. Get the rides & demonstrations on this level out of the way early & you will find that the line ups are fairly short, since most people work their way through the upper level first. When you're done at the lower level, you can head back "up" & do the tram tour & rides in this area.
Don't let the line ups for the tram tour discourage you when you arrive. They can put a lot of people onto one tram, so the lines move very quickly. I think the longest we waited was 10 minutes. Make sure you take little ones for a restroom break before doing the tram tour. It is about 45 minutes long and they don't let you get off anywhere along the way. You will find restrooms near the entrance to the tour.
At Water World, seat yourself in the areas marked "dry" unless you want to get completely soaking wet.
Our favorite things at Universal were Jurassic Park, the Mummy, Shrek, Terminator and the tram tour. A lot of the rides at Universal are actually 3D movies, rather than thrill rides, so it is a good place for folks who don't like the kinds of rides you'll find at Disney. But if you're not into thrill rides, stay off the Mummy! It is the wildest ride you'll find at Universal.
if you are planning on walking through the shopping area at the end of the day, don't bother paying for the preferred parking at Universal. The cheaper parking is at the far end of the shopping area and so you will walk right to the parking if you walk through the mall area.
The mexican restaurant in the Studio Walk is awesome!
While in this area, you may want to take a detour to Hollywood Blvd and see Grauman's Chinese Theater, the stars on the Blvd, and the Kodak Theater. You can get to Hollywood Blvd from the 101.