I think the trees look a little like cardboard cut-outs. Try and give the trunk some curvature so it looks like a 3D object. You've got light sources on characters and bushes, but other areas don't seem to have any.
For the water, I'd suggest adding a "ledge" to the top bits, to give it an impression of depth. Like this:
Thast's just a super quick edit, the colours you'd probably want to choose a little more carefully than those, but by giving the water that kind of "dip" it makes it a more tangible place for the water to be staying.
Hope that helps, you've got a good start here