This is a really simple way to get out of a creative rut. Pick a subject or theme and go with it. For example a common one is to pick a colour and take your camera and shoot nothing but things with that colour. Red is a popular choice for many people. Another possibility is to pick a geometric shape like a circle or square and find examples of that shape to photograph. Another could be symmetry. Look for repeating patterns. Take photos of an event in progress and put them together like a triptych. This simple technique is very effective and also developed your mind to see the world in a different world. One you have tried these different themes start to come up with your own, and then combine them is various ways. Only you can limit your possibilities for creativity. I would say the sky is the limit, but even that is not a limit.