Painted 2D Forest is the best package of quality assets to start:
-Trees
-Bushes
-More trees
–Big tree
-Funny tree
-Stones
-Platforms
+ mushrooms
+ glowing flowers
+ parallax background

This contains DEMO scene based strongly on Unity’s 2D Platformer

Note:
This pack is graphical assets in png large format. The demo scene is just for interactive preview.