We didn't tell our girls 2 years ago. Although we usually go at Christmas, we booked to go the day after our daughter got her GCSE results as she had worked so hard.
We thought of the most obscure country around the World Showcase and chose Norway and that's where we told them we were going LOL
Well they were not very impressed I can tell you and we couldnt get them out of bed at the hotel at Gatwick, it was so funny!
I didn't put luggage labels on until we got to checkin (not unusual for me) and my husband told us to sort that out whilst he found checkin. He tipped off the checkin staff and they were fab.
We got our daughter to hand over the passports and tickets on the basis that as she was nearly 17 she should learn how it was done. The staff kept a very professional straight face and the man asked her if she could confirm her destination. She said 'Norway'. He replied 'I think there must be some confusion here, these are return tickets to Orlando'. In what seemed slow motion she turned to my husband who was nodding yes that's right. She promptly burst into tears threw herself into my arm, I cried, my youngest then started and my hubby trying to hold it all together. It was great, something we will never forget!
We then checked in went for breakfast and they got more and more excited as they relived all the things they would be going back to do.
And of course they did get to go to Norway
Good luck - however you do it they will love it.
Twinks