You don't necessarily have to create something new. Since you want to brush up your skills, I would recommend you to translate something that is translated already. Then when you are done, you can compare it to the other translation. See what is different, and try to evaluate which parts of the translation are done better by you, and which one are done better by someone else. If you translate something new, you have nothing to compare it to.
I have worked at a translation agency (sales department, not translation itself), and in my experience no translation is ever the same. Give 10 certified translators 500 words to translate, and you will get 10 unique translations. Let certified translators look at each other's work, and they will always find something to comment or correct.