A simple task but with parameters that can change to make it more difficult.
A nice starting point is counting your steps on a walk.
Breathe in for 4 steps
Hold the in-breath for 4 steps
Breathe out for 4 steps
Hold the out-breath for 4 steps
You can change the intensity by increasing the number of steps for each stage or walk uphill.
1) It is a form of mindfulness because you can't really concentrate on anything else when you are walking, focusing on the breath and counting. So it's a good activity to get yourself out of your own head.
2) It stresses the cardiovascular system without adding physical stress to the body. By decreasing the O2 in and CO2 out, there is an added internal difficulty (your heart rate will increase) without increased external load (walking causes less external load than running at a speed that would cause the same heartrate increase).
Start with 10 minutes on flat ground and then steadily increase.
If you start to get light headed then return to normal breathing.
Enjoy drowning in fresh air :)