Here’s one simple exercise that you can do anytime without the need of some special equipment. I tried it and even the Russian snipers use it, so it’s safe.
First thing that you have to do is to find a location where you will position yourself. When you do, pull up your thumb, so that it is about one finger length from your nose. Look at your thumb and ignore the surroundings. Do that for 10 seconds. After that, focus on an object that is far away, but don’t remove your thumb from its position. Just look through it (like it was invisible), and focus on the object in the distance. Do that for 10 seconds also.
When you do that, return your focus to your thumb, and do the exercise from the beginning. Do the repetitions for about 2 minutes. After that close your eyes and relax.
In the beginning, some people, if they haven’t done these kinds of exercises before, may experience eye tension or slight headache, but it is nothing to worry about, because it will stop with a few sessions.
Do as many sessions as you feel you need, depending on your eyes strength. Just take care to make long pauses between sessions (e.g. one in the morning and one in the evening) so your eye muscles could rest and develop while resting.