Get a step counter and aim for 10,000 steps a day. First it makes you aware of how much (or little) you're moving each day - you have a real number you can see and a target to aim for. Second it sets you a reasonable goal to achieve every day no matter how you're feeling.

It's good for your mental health as well as physical health. There is good evidence that people who do the equivalent of 10,000 steps a day are generally healthier on many metrics, and the benefits plateau at around 10k. And on a bad day, going out for a walk to hit your 10k can make a huge difference to your mental health.

It's a simple, achievable but impactful lifestyle change that almosr anyone can make.

Edit: while you can get a step counter on your phone (including privacy apps like Pedometer on F-droid), I'd go for a dedicated clip on simple counter. There is something about a physical object dedicated to the task that makes a difference to me sticking to it. Also if you walk around without your phone a clip on device will keep on counting.