Countryside

Set the Colors to 2 colors (similar shades of blue or another color if want to do night or a sunset sky). 

Select the Gradient Tool. Drag it up/down the canvas...

Your layer looks like this...

Insert a new layer...then rename it "mountains".

Select the Brush tool, set it to a bigger size (around 20 ), and increase Hardness to 100%...

Set the Colors to the default...

...then set the Primary Color to a light/medium grey...

Now draw the outline for your mountains, something like this (make sure you draw it off the sides...

Select the Paint  Bucket tool and click in the mountain area a few times to fill it in like this...

Your layers should look something like this...

Insert a new layer, call it "sun".

First select the Shapes tool... then choose the Ellipse shape and set it to draw fill only...

Set the primary color to a yellow...

Draw a yellow-filled circle  near the middle of your canvas ...

Go to Effects>Blur... choose "Gaussian Blur"...

Play with the blur slider amount til you get something like this...

Then move the layer down below the mountains layer.

Select the Move tool...

Click on the canvas and then drag the sun to move it to where you want it behind/between mountain peaks...

Then get rid of the dotted outline (CTRL+D).

Insert a new layer, call it "grass".

Set your color to green.

Select the Brush tool. Set the thickness to 50 and hardness to 100%.

Paint the grass/hills near the front of the canvas, try to make it like grass, not totally flat/straight. Make sure you go off the sides...

You'll see it in your layer...

Go to Effects>Distort>Dents...

Play with the sliders to get something like this...

Insert a new layer, call it "cloud".

Set your colors to default and then switch so the primary color is white

Select the Brush tool again...

Draw a cloud shape (make sure it's in the middle of the canvas, not touching/off the edge of the canvas)

Then go to Effects>Blur... and set to "Fragment"...

You can leave it with the default settings to get something like this (or play with the sliders/amounts a bit)...

Duplicate the cloud layer 2x (so you have 3 clouds)

With the top cloud layer still selected, click on the Move tool...

...and click & drag on the canvas to move it over

CTRL+D to get rid of the selection/dotted outline.

Select the 2nd/middle cloud layer and drag it over somewhere...

CTRL+D to get rid of the selection/dotted outline.

Select the original/bottom cloud layer and drag it over somewhere...

CTRL+D to get rid of the selection/dotted outline.

Insert a new layer - call it "tree"...

Set the primary color to brown

Select the Trapezoid Shape tool.

Click and drag out a trunk shape on the left part of the canvas - make sure you're not drawing off the edge...

Go to Effects>Distort>Twist...

Play with the settings...

Make your trunk a little bit twisted...but not too much (should still look like a tree trunk).

Then insert a new layer above the trunk and name it "leaves"...

Change your primary color to orange or another color you want for your leaves.

Select the Brush tool. Make the size 100.

Click to create the leaves shape so it is not too smooth, has "bumps".

Try adding an effect to your tree leaves under EFFECTS>Distort>Crystalize...

Duplicate the tree trunk layer.

Use the Move tool and move it over to the right. Also make it a bit smaller (push in top-left corner).

Deselect the trunk copy layer. 

Duplicate the leaves layer

Move and resize the leaves copy.

CTRL+D to deselect.

Change the color of the 2nd tree leaves. Go to Adjustments>Hue/Saturation...

Play with the Hue slider. You can also adjust the saturation/lightness sliders to get the color you want.

Lastly, you can move the first tree over more to the left if it's covering the clouds. In my example I moved the trunk down off the front and the leaves off the side, and stretched the leaves to make them taller.

Add a vignette effect (under Effects>Photo>...) to the leaves and trunks.

Play with the settings so you have subtle shading effect.

Move the right tree maybe more to the middle so it's not covering that cloud. Also move the trunk layer down below grass.