This one may look easy, but it's really effective. Start standing facing one another and extend your arms out in front of you. Keeping your arms extended, one partner pushes into the other partner's hands. Both of you are providing resistance and pressing into the other. You will feel this in the backs of your arms but also your abs and midsection. Hold for 10 seconds and then switch sides, still facing each other. Alternate direction of push for a set of 10.