We did a water park. I hate water parks. To me it seemed like a wasted day especially since we live within driving distance of a water park Mecca. I would have rather spent more time at Magic Kingdom or Epcot.
We had five day passes. We spent a day at Magic Kingdom and saw about 1/4 of it. We spent a day at Animal Kingdom and saw 90% of it. We only spent 1/2 a day at Epcot because we had done the water park in the morning. We saw about 1/2 of it, but it was rushed. We actually spent a day and a half at Hollywood Studios. We had to go back a 2nd day because we didn't get a spot in the Jedi Training School which was the one thing my daughter wanted to do. One day would have been enough. We wasted an evening at Downtown Disney.
After talking to several people, we decided the meal plan would be a waste of money for us. We didn't want to do a character breakfast and didn't really care about eating at any of the themed restaurants. With younger kids, we knew they wouldn't eat enough to equal what the daily amount would be. We were able to eat lunch at one of the themed restaurants by stopping by just as they were opening and had an early lunch.
Definitely take advantage of Fast Passes. That's the only way you are going to get on some of the more popular rides. But, if you show up for a ride and there isn't a long line, don't waste your Fast Pass. You can trade it for a different ride. We had gotten Fast Passes for one ride, but when we got there it was only a 5 minute wait. We quick changed our Fast Pass to a different ride (using the My Disney Experience app). We actually got to ride that ride about three times never waiting for more than 5 minutes and got to ride a different ride that was one of our "if we don't have to wait in line" choices.
Park Hopper passes are worth it. I didn't think we would use them, but we did spend a day at one park and then went to another one in the evening.
Start with one of the smaller parks first. Magic Kingdom and Epcot can be really over whelming. Start with one of the smaller parks to get your feet wet and kind of get a feeling for how Disney works.
Personally, I wouldn't do Disney and Universal in the same trip. If you do, I would just get 1-2 day passes for Disney and just do Magic Kingdom and then do Universal. Five days of Disney was overload for me. We broke up the week by taking a day trip to NASA. Even then I was exhausted from the trip and swore I would never go back to Disney. I can't imagine driving home after a week at Disney. We just had an hour long drive from the airport to our house and I thought it seemed twice as long.
Lots of people told me to research as much as possible before the trip. I bought a book and tried to plan out every little detail. I gave up. It was just too overwhelming. Pick the top 10 things you want to be sure to do, then let the rest just happen. It will be a much more relaxed trip.