What makes a mobility perfect? Well, this depends on you only, most of the time.
What do you have to take into account before choosing your mobilities? Well, what you like to do! You like medicine and/or have a medical background, then choose a medical mobility (such as medical helper). If you like children, volunteer in a daycare.
Beside choosing your filed, it is important to have nice people to share your work with, but don`t worry, usually, volunteers are very social, friendly and empathetic.
So, before choosing your next mobility, make sure that you envision the future. A little bit at least, so you can make the best out of it!