Trees: If you go to a forest or such (That is not planted by hand) You won't find the trees in perfect rows and columns like the way you arranged them. Rather, it would look more randomized (If you look carefully, your trees make boxy edges).
As for the tiles, I don't work with custom tiles, so I can't really give you any links, but try to find tiles that flow together (if you look at your sand tileset, fourth picture, you'll see that the sand seems to abruptly change into dirt instead of flowing/blending into dirt. In other words, the sand is clashing with the dirt. Also, aesthetically, grass wouldn't be able to live next to ocean water due to salt, (reason why beaches are sandy or rocky).