Hi Nayomi!
I love this question! There are so many great ideas for dressing up which don't necessarily involve costumes. Not sure if you have heard of Disney Bounding but if you haven't you have to do an Internet search! Disney Bounding involves styling clothes and accessories to match a Disney theme or character. Check out this Disney Blog:
Disney Style. There are also some great ideas and pictures on Pinterest. You can also pick up some accessories such as the must have decorated and embellished Minnie Ears while at the Walt Disney World Resort!
As for the Anna and Elsa meet and greet, my daughter was able to take a photo with both of them, however it was very late at night and there was no wait. When there are lots of people, the have guests take a photo with one and then the other just to keep the line moving. I would highly recommend that you either get the FastPass+ selection for this experience or try to go either when the park first opens or right before it closes when the crowds are very low. Also, speak to the Cast Member present with the characters and let them know that you would like a photo with both, while its not guaranteed they may be able to help you out.
Hope this helps Nayomi! It has been a pleasure assisting you and I hope you and your friend have a fantastic vacation!
Thanks for sending your questions to the Disney Parks Moms Panel!
Yael