Side Tap
The side tap or hip tap push up is a great way to work both the upper body and also the core muscles, that will need to help fix your position when your on 1 arm.
You can either work one side at a time, or alternate arms, with those wishing to make the exercise even harder, you could hold suitable dumbbells in your hand.
Intermediate Level may decide to work this movement with the knees resting on the floor, as this will both reduce the workload on the upper body, and also reduce the use of the core muscles.