It depends on your child. My 10 year old with autism is happy with small plushes of his favorite characters at the time, any of the spinny light up things they sell at night and more than anything he loves pressed pennies. He likes to put them in and out of the holder, sort them in about 10 different ways (don't ask me the reason for the different sorts, havent figured them all out yet) or he's happy with a tshirt. We get it a size or 2 bigger and he loves sleeping in his big comfy shirt with shorts or sweats.
It depends on where the child's interests lie. You could also look for something that can be fun but therapeutic. For example, years back when my son was having trouble drinking out of an open cup we bought him cups with different characters. We let him choose which one to try with that day and he was excited about working on the skill instead of getting frustrated with it.
I think it's so nice of you to think of your neighbor. I would check with the parents and ask if she has a favorite character and what kinds of things she likes. Maybe she likes playing dress up and you can find some cute little necklaces/bracelets with the princesses or something. A few years ago I got a bunch of that stuff reasonably priced for my niece.