Thanks for all the ideas. Unfortunately, my sister wouldn't let her have many of the things mentioned. If it makes noise, a mess, or has too many pieces, she will get rid of it. I'll keep watching for more ideas.
In that case, books or dress-up things in a suitcase are probably your best bet. Who are her favorite characters?
One of my DD's most treasured possessions is a backpack of her very own. It is a clear one that is small-sized, and she takes it everywhere with her, carrying all her various treasures in it. She also loves a small rolling suitcase that used to belong to her brother -- her favorite pretend game is visiting the airport to go on a trip. She likes to pack and head out the door to the "airport" (swingset) in the back yard.
My DD does have Crayola Color Wonder paint brushes with the paint IN the brush. These are a bit hard to find, but they are the least messy "art" type things on the market, and IME kids love them because they can "paint" without an adult hovering. Be sure to add extra paper, because the little ones tend to just scribble, and that special CW paper gets used up very fast.
http://www.amazon.com/Crayola-Mess-Color-Wonder-Paints/dp/B0012N9OHY
Experiences are great, especially for a child with a younger sibling, but kids that age really don't understand gift certificates, so if your plan is to give her a special outing with her Aunt after her birthday, give her something small to unwrap anyway, and then invite her only a day or so before the outing will actually happen. My DD turned 3 last week, and to her, anything in the past is yesterday, and anything in the future is tomorrow, or if it is far in the future, it is Saturday.