I'm a product designer so I use it for quick early prototypes of parts that I design. This allows me to get better insight in the visual and mechanical properties, and to some extent test the products in actual use. I also do some 3d design, more artistic work, for which the 3d printer allows me to see how the object looks in real life, with all the imperfections that the 3d printing process introduces to the object such as layer stepping and support structure removal marks. I can then adapt the model for example by aligning certain details and lines in the model better to the lines created by the printed layers, or to change the scale of certain details so they come out better.

If you really can't think of anything at least you can experiment/hobby/toy around with it. Create a few objects every week, and I'm sure through such creative processes you'll naturally evolve a better idea of what you could do with it. Creating toys is an excellent purpose for 3d printers at the moment - design some interesting new toys and sell them to parents you know, for example. Or you could experiment with the technical side of 3d printing and help the community grow by developing better 3d printers for more and more affordable prices.