I so agree with going to the lower level first!
We did this in April, when spring break crowds were at a peak.
We went straight to Simpsons, and walked right on the ride.
On weekdays the lower lot opens 30min after the rest of the park, so we had a 5 minute wait until we could access the lower lot.
We went straight to the Mummy and rode 3 times in a row (it was so much fun)!
Then we rode Jurassic Park, and checked out some shows on the lower lot.
Heading back upstairs, we went to the Terminater show, followed by water world.
That was all done before lunch.
After that it was no problem. The park was busy, but we had no waits since all the big rides had been done already.
About 2pm we headed to the trams and waited 5-10 min for the tram tour.
One day is plenty, have fun!!!