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A secret garden
Imagine your own secret garden. Think about things it might have in it. Is it a happy garden? A creepy garden? Is it formal or wild? Where are the hidden entrances and spaces? Does it have a fountain or a pool or a well or even a cave? It's just for you. Make it a place that you would want to spend time in when you want to be alone. Then, don't tell anyone what the drawing is about. It's just for you!
A cityscape
Build your own city or town! Get out a ruler, or even cooler tools like a compass and t-square and learn about perspective. There are some interesting online videos about it. Then, play city planner and make up a town or city or neighborhood with really cool stuff in it. You could even get weird and draw in a giant or even elves and fairies peeking out of gutters.
An illustration
Think of your favorite fairy tale or book and draw the main character in a new situation that you make up. Like fan fiction, but drawing it instead of writing it. For instance, if you picked Harry Potter, you could draw what would happen if he met Voldemort on a beach. Or you could imagine what it would be like if Cinderella met Pocahontas. Or, you could illustrate a book or story that's never been illustrated or you could draw what Pocahontas or Cinderella would have REALLY looked like.
An animal
But don't just draw a unicorn or your dog. Draw the animal feeling an emotion or doing something active. Draw an animal for fun or for fun practice, not to see if you can draw the animal. Drawing isn't about getting it right the first time. You learn to draw some things by drawing them over and over. Sometimes the emotion you get from a picture is more important than if it looks real. So draw an animal doing or feeling something. You could even make it funny!
A tree or plant
You don't need to look to a real tree or plant for inspiration. When you draw this tree, think of yourself as the tree and what kind of tree you want to be - not the kind of tree you imagine yourself as now. Maybe your tree wants to reach to the sky or maybe it's big and full like a Christmas tree with lights. It might not even look like a real tree, it might be a fantasy tree. Think of what kind of tree you want to be and then draw that tree.
An underwater world
Draw a scene under the water. Maybe it's a dog swimming underwater in a swimming pool or it's a platypus looking behind rocks in a muddy pond. Or maybe it's even a giant squid having a party with merpeople at the bottom of the ocean. Use your imagination and make an interesting underwater world!