I'm snuggled in under my heated plush throw and following links on Ruth Armitage's blog, and just found a wonderful example of a series. It happens to be on a theme I really love—rivers. It's the River Series by Casey Klahn. The individual works are all deliciously unique in composition, and a great lesson on how much variety you can achieve within a constricted framework.
I suspect there is no end to creativity. If you try to squeeze it from one direction, it just spills out in different ways to the other. The more you try to focus it down to one idea, the more variety you find popping up within that smaller idea. If you think about just the obvious elements of a painting, it's not hard to see how by varying just one or two things, you could get dozens of variations:
1. Subject idea
2. Overall design
3. Dominant colors
4. Emotional content
5. Painting style
You could also use a series to try on different styles. Pick a favorite photo or painting of yours and try painting it as say, five of your favorite artists. Pick artists with styles very different from yours, and different from each other.
Something I'm doing right now with my sketches is playing with other styles, incorporating lines by drawing with the brush on top of my painting, copying the color schemes others use, and incorporating abstract blocks of color in otherwise representational paintings. So far, they haven't been successful enough to share, but they are stretching my brain and I find myself thinking of approaching my compositions in ways I never have before. They're making me focus more on the painting as a whole, and less on the subject. I touched on this idea in "The reference and the subject", on my desire to do less copying and more innovating. It's tough to innovate when you don't know how to come up with different ideas. When I feel like I'm stuck in a rut, I want to be able to remember that there are different languages of visual expression, and that I know a few words in some of them. The other thing I want to always remember is that I can combine those words in any way I want, as long as it makes something I like.