I suprised my DS, who was 4 at the time. It was a trip just for him and I so, I told him about a week in advance that we were going to spend a day together, just him and I. We drove to the airport, telling him that daddy and his brother were going to watch airplanes take off and land while we went to the zoo. Well, when I told him that he and I were actually going to Disneyland, his first reaction was, "Do we have to?" LOL! It took about a minute to sink in, but then he was jumping around saying thank you, thank you and was really excited. At the end of the trip, he did ask daddy if he and his brother had fun watching the airplanes the whole time we were gone! LOL
For a first trip, I would want to include her, at least a few days before hand, so she can build up the excitement and help "plan" a few things. Count downs are really fun for that age group. First trips can be a little overwhelming too, so it would be nice for her to get into the Disney mode, as it were.