Get down on your hands and knees, and then extend your legs so your body-weight is supported by your palms and toes, and your back forms a straight, horizontal line. Hold this for as long as you can. Thirty seconds is a good start. Two minutes is a pro level killer.
More and more parks include exercise bars these days. But anything will do, from a sturdy tree branch to any safe, horizontal bar (although don’t try it on public transport). With your palms facing away from you and your hands slightly wider than your shoulders, squeeze the bar, focusing on contracting your shoulder blades as you lift yourself from the ground, then slowly lower. Don’t be dismayed if you can only manage a handful of pull-ups at first; this is a notoriously difficult exercise, but fantastic for stretching out your back.
An easy one. Stand with your back flat against the wall, as outlined above. Hold your hands up, so your elbows are at right angles. Your arms and head should form a ‘W’ shape. Without moving your hands from the wall, push them up so your arms form a ‘Y’ shape, then slowly lower. Do 10, then relax.