Here's a seated medicine ball toss (also called a seated oblique toss) for two people that can really work your core muscles. We used an 8 pound medicine ball.<
Sit facing in the same direction or opposite direction a few feet away from your partner. In the pictures below, Dan Motowski and I sit facing away from each other. The knees are slightly bent. Key: Hold your transversus abdominis or TrA* muscle tight. Put the ball to your side away from your partner, rotate toward your partner, and throw the ball. Your partner will catch the ball and let his/her arms swing away from you, letting the oblique muscle brake the momentum.
After you've done a few, turn around so you can do the opposite side. Variation: you can lean back a bit to work the abs and obliques even more. Or get up and do a standing oblique toss (recommended if the ground is wet!).
*Don't know what your TrA is? Let out a "Hssssssss" and you'll feel it tighten. If you really want to know more about the TrA, ready Douglas Garfield's excellent book, "The New Steady Ski for Nordic Athletes."
Pictures by Team NordicSkiRacer.com's Mike Heidinger.