Hi there, Leila!
What an exciting vacation you've got planned with three generations of your family! You are all going to have an amazing time!
I'll be honest with you, some of the most special experiences my family has had at Walt Disney World have cost us little to nothing extra. There is magic happening just about everywhere you turn no matter which Park you're visiting. Since you've got a little Princess, I'd suggest buying her a princess costume at home along with some accessories to go along with it. I've been able to find so many fun things at our local dollar store! You can pack the costume and accessories secretly and they can "magically" appear in your hotel room for her to get all dressed up! You can even buy a couple of different ones if she's got more than one favorite!
Something very special happens at Magic Kingdom Park when you arrive just a few minutes before the Park opens for the day. There is a wonderful way that they open the "gates" each morning with a song and lots of Characters! It's a most magical way to start your day, you won't want to miss it!
One thing my daughter still enjoys is a visit to
Harmony Barber Shop on Main Street USA in Magic Kingdom Park. They will sprinkle your daughter's hair with Pixie Dust for just $5 and they put hairspray on so it will last awhile! It's really cute and you don't even need an appointment!
I hope this gives you a place to start, Leila! I have lots more ideas, but I'm just about out of room for my answer. Please don't hesitate to write back in if you'd like some more!
Wishing you and your family a Pixie Dusted vacation!
~Jackie