One thing that can really stretch you is to choose to shoot only one thing, or to at least do a series on one thing – i.e., a year of door shots, a week of only shooting the number 7, or only taking pictures of the backs of heads, etc. Make it as challenging or simple as you want. I’ve seen studies on hands (which is always a good topic, because so much of a person’s life is written on them, but also kind of overdone at this point), windows, leaves, etc. One day, just on a lark, I spent the day shooting things on the ground that looked interesting. There were feathers covered with dew, some bugs here and there, a piece of trash, a trampled flower, etc. It was the wrong time of day for really fascinating light, but the still available dew did add some additional interest.
Whenever I get a new lens, it stays on my camera if at all possible for at least two weeks before I go back to swapping lenses around again. It gives me a really good feeling for the lenses capabilities and forces me to sometimes frame things differently than I would have if I had a more convenient lens to use for the image. This is especially true for prime lenses where you have to zoom with your feet instead of the lens!