If you always run (or walk) the same route every time you run, next time, run your route in reverse.
No, I am not telling you to run backwards, but just run in the opposite direction. For example, say you always run from your house to the park, around the pond and down the big hill back to your house; this time run up the big hill from your house, around the pond, through the park and back to your house. Make sense? You'll be totally amazed at how different the run is, especially if your run usually consists of going downhill, an uphill run will really rev up your workout. Your muscles and brain will welcome the change.