The cloud study is a great plein air exercise. It's something you can do anywhere, even in the city, or in a landscape where the scenery at ground level is totally uninspiring.
And because clouds are a moving target, it forces you to paint rapidly. Having a palette prepared with tonal strings of blues and greys would be a good idea, so that you don't waste time mixing. Adding a speck of yellow to the whites and violet to the grey shadows produces a nice complementary colour harmony.
Johan Dahl, Cloud study (Wolkenstudie)
Views of clouds reflected in water are an interesting variation, popularised of course by Monet.
Here's one by the Western Australian artist George Haynes.