When the kids were little and we would drive off from his Mum and Dad (whether it was at their house or someone else's) we would wait until we were about to drive forward, have all of the windows down and co-ordinate a very loud, long "Bye". They get a bit embarrassed now as teenagers but it still happens if it's not too public.
That also reminds me of a tradition that my husband's father started with my girls by accident. Sadly, he passed away earlier this year (small cry, here) but it will always be remembered fondly. He thought that he was being playful with my eldest daughter when she was about 18 months old, and he growled at her. She looked startled and terrified and my FIL was devastated. My husband quickly jumped in and told my daughter to growl back at him. Bravely, she did and my FIL caught on and looked suitably frightened in a jokey way. That became their habitual greeting to each other, taken up by my youngest daughter too, when she was old enough. (very teary, now - reaching for the tissues)
