This has been recommended to me and a lot of people I've known with PTSD use it.

The Five Senses Exercise
One example of a mindfulness exercise is called the Five Senses Exercise. The goal is to calm your mind by using your five senses to focus on your environment instead of your thoughts.

Here are the steps to complete this exercise:

First, notice 5 things that you can see. Look around you and become aware of your environment. Try to pick out something that you don’t usually notice.
Second, notice 4 things you can feel. Bring attention to the things that you’re currently feeling, such as the texture of your clothing or the smooth surface of the table you’re resting your hands on.
Third, notice 3 things that you can hear. Listen for and notice things in the background that you don’t normally notice. It could be the birds chirping outside or an appliance humming in the next room.
Fourth, notice 2 things you can smell. Bring attention to scents that you usually filter out, either pleasant or unpleasant. Catch a whiff of the pine trees outside or food cooking in the kitchen.
Finally, notice 1 thing you can taste. Take a sip of a drink, chew gum, or notice the current taste in your mouth.

Mindful Monday: Try the "Five Senses" Mindfulness Exercise