Spiderman
The spiderman push up, is a great movement to work your upper body - core and hip flexor muscles.
Many people perform this exercise, by bringing the knee up towards the elbow of the same side, however allow their feet to touch the floor in this phse.
Focus throughout to smoothly bring 1 knee up towards the elbow, whilst keeping the foot off the floor until you have extended the leg back.
By doing this you will help keep the contraction within the muscles being worked, and as such get better results.
To assist your balance, aim to have your arms and legs slightly wider than normal, as this will reduce the exercise difficulty.