Just got back with my just turned 5 yr old. You could check out my trip report over on that board, but there's lots to do!
The first thing I did when deciding to go or not, was to go to Universal's site and print out a list of all the attractions
http://themeparks.universalstudios.com/orlando/website/ioa_attractions.html
http://themeparks.universalstudios.com/orlando/website/usf_attraction_more.html
I crossed out the 5 rides ds was to short for, put stars next to things I knew he'd love and then everything else...I had him look at the clips on the website to see if he thought they were too scary or not interesting or fun etc, asked or searched about specific rides here and I got this book which was highly recommended here
Kelly Monaghan
Author of "Universal Orlando: The Ultimate Guide" and "The Other Orlando: What To Do When You've Done Disney & Universal"
and read the detailed descriptions, which gave me an even better idea.
My ds is pretty fearless, so it was an easy decision really, but I have an extra sensitive 6 yr old nephew who would not have enjoyed himself nearly as much, so it is worth it to do some homework when taking little guys.
The Curious George area and Seuss Landing alone would probably keep ds happy all day, but we have to save and plan for our trips, so I wanted him to be able to do most of the attractions and it did end up being worth it for us. We had a blast and went back for a quick trip two weeks later.
Hope this helps...makes sense, LOL.
Have fun planning.