All you need is a 3 inch PVC pipe - either cut one up or just purchase the precut size, which is what I did. Purchase two 3 inch PVC pipe caps.
Then purchase a board - since I was making multiples of this I purchased one long board and had it cut into two foot sections. I chose a 1x10 inch board.
If you wish to make it harder you can switch to a larger PVC pipe and/or a slightly shorter board.
You can also decorate the board and/or the pipe. Without decorations not only does the board require balancing left to right but the wood on the hard plastic slides back and forth. That dual level of instability is difficult for smaller kids.
I share my projects at some great link parties.
You can see them all HERE