I would have to differ in my opinion about the park hopper option only because of the time you are going. I think it is worth getting especially during a low month like Sept. Lines are not long, you get through the parks a lot faster than say a more crowded month. You are more flexible on your options especially if you want to go to the Magic Kingdom during the day, relax at hotel and then have dinner at the world.
Dont get the water park add on.....especially in Sept.....you never know what the weather will do in Hurricane season. If you want to go just pay day by day.
I 2nd the motion, Dont spend the money on Spirit of Aloha, the food is not worth the money...
Also, avoid DisneyQuest not worth it.
-in my opinion, if you are staying on property at a moderate, avoid the sit down restaurants not worth the money, either use the food court, or go out to the parks or Downtown Disney
-buying ponchos or umbrellas...easy bring your own poncho that costs $3 compared to $15
Musts besides the attractions...
-have lunch at 50's primetime cafe or sci-fi at HS
-if you are staying on property (which I highly suggest) refillable mugs...not only do you use it there but consider a usable souvineer at home.
-if you like margaritas! I highly suggest you go and visit the La Cava de Tequila in Mexico....worth the $Margarita!
-visit the Christmas shop at Downtown Disney
***worth the extra $$ Disney will ship purchases to your home, so that you don't have to packem on the plane***
-eat dinner anywhere in the world, EXCEPT Morocco!
Thats all I can think of right now. Again this is just my opinion!