Not my dad, but for my slightly-too-tubby mum, me and my siblings bought her a pedometer one year for her birthday, together with a set of kilometre targets to make 'Mum's walk around the world'. At each target (planned for about once a month for a year) one of us would take her out for a reward - a Thai massage when she had walked as far as Thailand, a home-cooked Spanish meal when she made it to Spain. She was grumpy when she first got the present and thought we were forcing her to exercise, but actually then really liked the guarantee of seeing us at least once a month.