Fitball

My favorite hobby nowadays would be fitball. When I am stressed out or I have a bad day, I usually go to the gym and it helps me to relax, to get into a good mood and feel better.

I especially (I'm pretty sure you can use specially or especially here, but to my ear, especially sounds correct) love fitball classes, because the way I see it, it is a great way to work out (workout is a noun, as a verb you should say "work out") every muscle in your body. The activity consists of doing exercises with a ball which is an unstable base and it helps me to improve my balance and to strengthen my muscles.
To practice this activity you only need to wear gym clothes, a ball (if you don't remove "to wear" it sounds like you are wearing gym clothes & a ball) which you can get in the gym and a small towel to wipe your sweat. Moreover you should not forget water because it is important to keep hydrated while you are exercising. It is better if your T-shirt is made of breathable fabric like cotton.

I love fitball and I enjoy practicing it in my spare time. I would recommend everybody who is interested in sports to do it at least once in his/her (come on now, let's be inclusive for all, haha) life because everybody can do it and it is very fun.

Wow, very good. The corrections I've made are very minor. 

Fitball

My favorite hobby nowadays would be fitball. When I am stressed out or I have a bad day, I usually go to the gym and it helps me to relax, to get into a good mood and feel better.

I specially love fitball classes, because the way I see it is a great way to workout every muscle in your body. The activity consists of doing exercises with a ball which is an unstable base and it helps me to improve my balance and to strengthen my muscles.

To practice this activity you only need to wear sport clothes, a ball which you can get in the gym and a small towel to wipe your sweat. Moreover you should not forget water because it is important to keep hydrated whilst you are exercising. It is better if your T-shirt is made of breathable fabric like cotton.

I love fitball and I enjoy practicing it in my spare time. I would recommend that everybody who is interested in sports to do it at least once in her his/her (or their) life because everybody can do it and it is very fun.

Overall, this is excellent. HIs/her vs their is tricky. See a good discussion at http://www.merriam-webster.com/video/the-awkward-case-of-his-or-her.