Question about the balloon donating feature. Which balloons are accepted? Is it any? Morostide balloons? New Years balloons? Random balloons? Or is it only carnival balloons?

I think they only accept balloons which have been restocked at the Balloon Hut, this is a part of the news post:

The only requirement is that the balloons are actual balloons and were items once available from the Balloon Hut.

What do you get for donating balloons?

There's an achievement The direct link doesn't show your progress, in case you've started on it, just so you know