I bet they try to one-up each other on push-ups. Harley can do them one-handed *winkwink*
His military upbringing was a little bit different than her's but I think mostly the same. They can bond over having the same sort of family and training. He was actually in the AirForce so that's what he knows best. Get him talking about planes and he'll go on for hours. Harley isn't too great in the mental department either but he's never been all that close to people most of his life. Negan being the exception and I can imagine maybe Cee once they got to bond and know each other better.