Everyday, I do 100 pushups - 50 when I wake up & 50 before I go to bed. It takes 2 - 3 minutes from your day so you have no excuse. If I have extra free time I'll go swim at the local pool.
Adam Savage (of Mythbusters fame) does a really awesome podcast on which they discussed fitness/well-being one week[1]. The main takeaway is in order to become and more importantly stay healthy you have to change your mindset to always be thinking about what you eat.