Well, this isn't as grand of a surprise as most of you are planning, but I've kept the secret for 3 months now and I'm about to burst!
We're going the end of Sept/early Oct, and we'll be there on dd's 8th birthday. I got CRT for lunch on her Bday, and I plan to order a nice birthday cake for after our meal. I'd always planned on telling her 2 or 3 weeks before the trip, because she's the type who really needs to plan ahead, watch videos of certain rides before she's comfortable going on them, etc. We're sending her an invitation "from Cinderella" to celebrate her birthday over lunch at the castle, with the princesses. Well, I just couldn't take it any longer, I put the invitation in the mail today. I have no idea what her reaction will be... she knows the characters aren't real, and she may immediately realize we've planned a trip, or she may think it's a real invite, and beg us to go.

:
I've also been talking way too much about it to our 2yo. At first, she was pretty oblivious about the whole thing, but it seems to have clicked over the last couple of days. She's told the cashier at the supermarket that she's going to ride a train and meet Tigger. Last night she said "I want to see Cinderella", and luckily, dd7-almost-8 assumed she meant the movie and just put the DVD in.

I hope that invitation comes soon, before little sis blows the surprise!!