WOWWOWWOWWOW
What a fantastic thing to do.. DH is playing footie right now.. but the second he gets back i'm gonna show him this!!
I even had to make sure he had booked my birthday surprise tea at the ritz
I'm undecided on whether you should wait until the last minute to tell her about the trip.. I agree it would be a fantastic surprise to be told on the way to the airport.. I guess only you will know if she would like that kind of surprise!!
BUT from reading these boards, a lot of people get sooo much fun from the planning/looking forward to going, that it maybe seems a shame that she wil miss out on that?
Maybe you could introduce a few hints in the week leading up to the trip and like someone else suggested I'm sure presenting the tickets.. with a bottle to champers in celebration

... a few days before you go will give her the opportunity to really get into the trip before you go?
Anyway whatever you decide i#m sure you will have just the most fantastic time