It depends what you want to do. If you want to test formatting features, you can type in the question editing box or the answer editing box and look at the preview, refraining from posting. If you want to experiment with editing, commenting, etc. then there is no sandbox where what you do doesn't affect other people, but there is a dedicated sandbox thread on the main meta site where you are free to experiment. If you wish to use math formatting, there is a similar thread on the Math meta.