A mate of mine found a book that was basically weight training stuff using your kid as the weight. This included leg lifts, pushups, squats, arms etc... Dad gets a workout and the kid thinks it's amazing having Dad lift him and throw him around. Everyone wins!

I am linked to http://sciencealert.com.au/ and http://www.ncbi.nlm.nih.gov/ which post things to my facebook news feeds occasionally.