The late, great Steve Jobs said something I found fascinating:
"Creativity is just connecting things. When you ask creative people how they did something, they feel a little guilty because they didn't really do it, they just saw something. It seemed obvious to them after a while. That's because they were able to connect experiences they've had and synthesize new things."
Creativity isn't about creating something new; it's about creating new connections between already existing things.
For an example, let's look at an excellent book series: Mistborn by Brandon Sanderson.
The Mistborn series is all about a young girl named Vin who finds out that she has "magical powers" that come from the most unexpected source: metal. When she eats metal, it gives her (and the others in her world) certain powers depending on the type of metal she consumes.
What an awesome concept, right? And yet, it's such a simple one! It's taking the metals that are found in our world (steel, iron, brass, etc.), but making a new connection between "metal" and "magic". All of a sudden, two completely unrelated things are linked in a fresh, new way.
Another example is Brandon Sanderson's Rithmatist series. In this series, chalk drawings can come to life and kill people. Say what? How are chalk drawings dangerous? You have a brand new take on something as simple as "drawing" and "fictional monsters".
If you examine 95% of the most popular fiction today, you'll find that very little of it is actually completely "new". There are very few stories that present totally new and unique concepts. Instead, these stories are taking things that we are all familiar with, and putting a fresh twist on them or making new connections.
THAT is the secret to creativity!
I will now refer (apologies) to my own work. In The Last Bucelarii series, the Hunter (the main character) has one weakness: iron. This simple connection between "iron" and "poisonous to demonkind" makes for a fresh take on something old (a hero needing to have a weakness). All of a sudden, iron becomes an incredibly important metal in the book, and just its presence can increase the tension of any given scene.
The takeaway from this post: don't be so focused on "breaking out of the box" that you go too far out into left field. You'd be surprised how creative you can be just by forging new connections between seemingly ordinary things.