If you are able to do a regular pushup without assistance, then perhaps a challenge is in order for you. The challenge is to do a regular pushup on the balance board. A balance board is normally used to stand on while practicing upright balance, however in this instance you will be maintaining a level board while doing a pushup on it.
For some this will be a piece of cake, for others not so much. The trick is to keep the board level when doing the pushups. This is a good exercise for the shoulders as you will soon see when doing them.