I don't make my livelyhood from graphic design. But if I did, I think it would be very hard to continually create someone else's ideas or vision. I have great respect for artists that can take a concept and develop it graphically, especially when it's not their own. I suppose in that case, the satisfaction comes more from being able to interpret ideas and then translate them graphically. That's an incredible talent to be proud of.
But for the times when we get to just free style our art, my advice is to start with what moves you. If you are writing a novel, make sure there are times when you have to look through tears to see your computer monitor. If you are composing a song, let the music express your most joyous thoughts or your most desparate fears. For everything you do creatively, try and capture the extreme emotions that are only yours, even if you never tell anyone what actually drove your creation.
I drove through Mill Creek Park last night and got some shots. This is Lanterman's Mill (with a few tweaks from Photoshop).