This is a really fun exercise using a big brush and completely limiting the amount of brush strokes you use. A household paintbrush will do if you don't have anything more professional.
I always think landscapes look great on a warm coloured ground like a wash of Burnt Sienna, a pink or a light orangey scarlet.
Find a landscape to paint. Whittle the information within the landscape into 12 colours and their shapes. Mix up a good dollop of these 12 colours to include the sky.
Pick up a thick dollop of paint on your brush and touch your canvas only once with each colour, you can move your brush around with each colour but don't take it off the canvas. If you do, you have to change to the next colour. It's a great exercise and produces surprisingly effective results...
If you feel the painting looks too unfinished, halve the size of your brush, mix 12 more colours using variations and tones of the colours you have already used and do the same exercise with the smaller brush. If you STILL feel it looks too unfinished, repeat...ad infinitum!