+1 for the swiss ball, along with weights and a medicine ball. Get a good book of core exercises and work on all the muscle groups in your core. Your core not only supports your body upright but also while you twist, bend and lift, so you need to do a variety of exercises, not just sit ups or the plank.
Pilates is good, too, although a lot of blokes won’t do it as it’s perceived as a womens’ thing. But I know a few blokes who are doing Pilates exercises – some given to them by physios without referring to them as Pilates. They look gentle and ineffectual but try a few – you’ll hurt afterwards!