The best internal workout you can give yourself is good breathing.
In through the nose expanding and filling the belly, then the chest right up to the shoulders. Then breathing out and reversing the process emptying the belly last.
When you do this you fill your whole lungs and you expand your diaphragm properly. You can feel it working all the muscles of your torso from inside - all those core muscles get stretched and worked. When the diaphragm is worked properly like this is massages all the internal organs and helps to pump your lymph which is important for fighting infection.
You will also notice that your sacrum, spinal column and head move with the breath in an 'S' shape helping to pump your cerebrospinal fluid and keeping your back flexible.
Good breathing will also help you to release stuck energy, particularly emotional stress which tends to collect around the diaphragm and belly.
Take a few minutes a few times a day and focus on your breathing. The more you do this the more natural good breathing becomes and the less stressed you will feel.