The next time you go for a walk in the park, practice doing it mindfully.
As you are walking, you might find that your mind is creating its usual array of worries. You are pulled into your head and away from experiencing the walk. Trying to push the worries away just seems to make them stronger.
Instead of fighting them, acknowledge they are there (“Oh, there’s that worry thought again”), let the thoughts be (not suppressing them or trying to fix things in your head), and then consciously bring your focus back to the present moment. You could notice the warm sun on your face and the cool breeze on your cheeks. You could look up and watch the birds flying above you or look down at the plants below you. You could notice the scents of nature in the air and the feel of the earth under your feet.
Every time your worries try to pull you away (and this might be every 20 seconds at first), gently let them go and return to being aware of the world around you. This is walking in a mindful way.
This is in contrast to how we often do things mindlessly. We may go for a walk, but we are caught up in our thoughts and are not really being aware of our surroundings. Luckily, we can practice being in the moment and develop much richer experiences of life. Any activity can become an opportunity to practice mindfulness: taking a shower, washing dishes, preparing and drinking tea, eating a meal.