Besides infinite wishes of course, I would wish for a ton of trees to grow all around the world. But not regular old trees, these would grow fruit that cures all sicknesses/ailments/diseases, etc., and supplies all the necessary nutrients for all organisms to live. These could also grow in any environment (Including underwater), so they could be planted anywhere with the seeds from the fruit. Also, these trees would grow only as big as the space it's in, they wouldn't grow into buildings, and it's roots wouldn't grow into pipes or anything like that. But it would keep growing if put in a huge empty space.
Imagine how efficient everyone could be without having to worry about how they are going to get food or if they are going to get better. :D