I used recycled plastic lumber for my stalls. I chose it because I did not want the horses to chew it, it looks similar to real lumber and can be cut with all regular wood tools. I love it so far. They don't chew it and it can be scrubed and washed down since it is impervious to water. It won't rot or mildew over time like traditional lumber.
It is much more expensive than real lumber though so I wouldn't use it as arena kickboards.
We used recycled rubber flooring in the alleyway. This you can not cut with power tools or it will melt. You can easily cut it with a utility knife which is what we did. This is a wonderful work surface for the horses.