In a 2D game you want to set a shadow direction from the beginning of making. I see you chose the light to come from the right side... I do the same thing in all games I make..
For the trees you might want to have the leaves on the left side a tad darker.. Also I would suggest a few stone tiles to throw aroudn in the grass in certain places. Stuff that takes 1 minute to make can really make the game feel more alive.
Some weeds, a bush, a few different types of rocks. These are the things that grab the player and keep them there..
OH btw...You see your alternate grass tile? The one right above the bridge... It took me a while to notice it...What I think you should do is add a slightly darker shade of green to some parts of that grass to make it seem taller and more tangled... It will be way more noticable.
Good luck !