I have DS not DD, but he is 4, and has been to WDW 6 times already. His most favorite place in the whole entire world!
Some things he has loved on the last couple of trips that should appeal to either boys or girls:
Characters, characters, characters! Especially our last trip in May when he just turned 4, the mission of the whole trip was about how many characters we could meet to get autographs and pictures.
Fav rides: Peter Pan, Small World, Dumbo, (did NOT love Tiki room this time, too dark and loud), Buzz ride, Monorail, Star Tours, Soarin', Safari ride at AK
Fav shows: Lion King at AK (he got to participate in the dancing at the end this time), Cindarellabration, watching the outdoor performers around Epcot, Playhouse Disney Live on Stage
I've heard that the Bibbity Bobbity Boutique at DD is a huge hit with the girls - we saw many Princesses walking in the parks enjoying the limelight. We actually let our son get a couple of costumes down there to wear at the parks too - he was Peter Pan for a few days (not sure why I bothered packing him regular clothes?!) and a Safari costume from AK was a huge hit too (both are still active in his playwear at home too). He got a little tired of the extra attention from people, but we thought it was hillarious. One girl even asked to take her picture with our little "Peter". Many girl costumes to choose from even if you opt out of the BBB experience.
Most of all, just let them have some time to be playful and active down there too. It is hard standing in lines all day (FastPass is a parent's best friend), so we let him play at the playground areas when we got a chance, and have some pool time at the resort either in the morning or end of the day.
Have a blast, and make some special memories that will last a lifetime (and kids remember more than you think they will at this age).