There really isn't a right answer, here. Everybody has something different that sparks their interest. Music is a common tool; personally, I find that the rhythm of certain songs really gets my blood pumping, and once I'm comfortable with that I start listening to the lyrics and it's like instant magic.
Movies work, too (this includes television shows). I'm not saying to watch something and then copy it's exact plot, dialogue, characters, etc, but I'm saying that certain situations can make you ask some interesting questions, and maybe make you apply those questions to your own novel.
You can hit the gym; I've been jumping on the treadmill (not literally jumping; that would be dangerous) and running for about half an hour these past few days, and not only has it helped clear my head amidst the onslaught of English papers I'm trying to write, but I was able to think critically about the role some of my characters play in The Hollow Men.
Don't forget how valuable the classroom is, either. The majority of my ideas have come from history lessons and books I've read in class. Don't be afraid to apply the things you learn to your creative life; it's those kinds of things that keep your work interesting. As an example, The Hollow Men was inspired by a class I took last semester, titled "Consumers and the Law." Don't see a connection? One of our lectures covered the Bill of Rights, particularly focusing on the first amendment (Freedom of Speech).
After this lecture, I began to think about what would happen if that right was taken away from us, and we lived in a completely militarized state.
What I'm trying to get at is that inspiration can come from anywhere, and it can strike at any time. The shower, driving, grocery shopping, the middle of your work day. The trick is to be prepared. Something I've always found is that my mind can work at its best when I'm doing some sort of mundane task (I've already covered the shower and driving, and I swear that's when I do a lot of my thinking about my books).
Don't ever doubt anything. When an idea strikes, don't shoot it down; think about it. Ask "What If." The possibilities are endless with that question. Case in Point: a few years ago I was having some back pains. I was walking into school and suddenly had a revelation: "What if I'm growing wings?"
I would have been stoked if that was really happening. But it didn't. Instead, I was able to transform that simple "What If" question into an entire book (then series) that I worked on for months. That idea is still one of my babies that I hope to return to one day.
So take a look around. Figure out what interests you. Apply that to your imagination and go crazy like a little kid with crayons and a white wall.