Well, if you all love rollercoaster, then both parks have two rollercoasters each that are not to be missed! The Mummy is AMAZING!
I would start my day out at Universal bright and early. They open at 9AM the whole month, so I would arrive at the parking lot no latter than 8AM because it is a long walk from the parking area to Citywalk. As you come into Citywalk, Universal will be on your right. Get into the parks and hit the Hollywood Rip Ride Rockit first thing! After than head over to the Mummy and ride that as well. Once the two big coasters are done, there are some more rides to still see depending on what you want to do. Men in Black is a must, Jaws is fun, Simpsons (if you like the TV show), and a few others. You would have to decide if you would want to ride stuff like ET or Shrek. I would also pass on Twister simply because of the amount of time it can take for an somewhat boring special effects experince.
No matter what you do, I would start leaving Universal around noon or 1pm at the latest. It will take about 5 minutes to walk to the other park. Once in there, ride Hulk, Spiderman, Jurrasic Park (if its not to cold) and then you will be right next to HP which has another roller coaster and of course the new HP ride which is mind blowing. So awesome!
No matter what you decide to do, you always have the option of purchasing the express passes at the park. DO NOT buy them early. Wait and see if the park is going to be busy and if so, then purchase them inside either park. These allow you to skip the lines at almost every ride in the park without having to wait for a return time like at Disney and normally without merging with any other line, its express all the way to the ride. These passes do not work on the main HP ride, the Hollywood Rip Ride Rockit, and a ride that you and your party would not even be able to ride because you do not have a child with you.
So should be able to see both parks main rides in one day even without the express as long as the park is not busy. Like another person said, if you are going at the start of the month, it will still be busy but if your going near the end, it should be fairly empty.