If you're just spending one day, Magic Kingdom is the standard. My kids enjoyed Animal Kingdom even better, and all the rest they simply tolerated. They're not very theme-park oriented. They like amusement parks a lot more because they like to go fast. There's not a lot of rides at Disney, there are lots of experiences at Disney.
Just go slow and chase your daughter. She'll tell you what she wants to see.
On the park hopper pass...
If you hit up the girl in front of Unimart or at the stands, etc., they'll sell you park hopper passes that just have a day of Magic Kingdom left on them. The passes are just like store debit cards. They load them with x amount of days at x amount of parks. As they get used up, they just show what's left. You shouldn't have any trouble getting Park Hopper passes for just Magic Kingdom. You may have to use them and drop them back to whoever you get them from because there will be other passes still on the cards, that's happened to me before. I just used what we used, stopped back, gave the girl the cards and paid her. I was really surprised how much trust they're willing to put out there, but it's nice.