I asked my 9 (almost 10) yo ds and he said "pretty much all rides are good". I had him look through our guidebook and this is what he came up with.
MK -Space Mountain, Mickey's Philharmagic, Buzz Lighyear, BTM railroad, WDW railroad, Tomorrowland Transit Authority, Tomorrowland Indy Speedway, Barnstormer, (He rode all the rides in Fantasyland with his then almost 4 yr old brother and didn't complain at all. We also waited in line to see Cinderella and get her autograph. No complaints. I think it's all on how you approach it too. At the time, my 4 year old loved the movie Cinderella.)
Epcot - Mission Space, Test Track, Living Seas (if he likes water animals), Innoventions, collect the mask and charms at each country,
MGM - Star Tours, Studios Backlot Tour, Indiana Jones, Toy Story Pizza Planet for the arcade, Fantasmic,
AK - He loved getting all the stamps in his brochure book at AK. You pick one up at AK and you can go around to all the areas and do little projects to get a stamp. When you get all of the stamps you get a big stamp in your book. It's really fun! Rafiki's Planet Watch (to solve the puzzles to get a stamp and to see some animals), Lion King, Kilimanjaro Safari, Primeval Whirl (if spinning doesn't make you sick)
He loved getting autographs from the characters and collecting the smashed pennies. Make sure and stay for the fireworks and try and see a parade! I agree with the water parks! Try and go to a water park. We all had a total blast there. They have areas for little kids too. We only spent half a day there last year and we plan to go for a whole day the next time we go!
HTH!