if u want to prepare them for primary school, guess would be language, drama, math classes.
if u want them to have skill, i think is best time to start swimming, martial arts, dancing and music. best to start these when they r free-er.
Music/piano/keyboard is great for working the brain and improving finger motor skills. Most kids start around 4 or 5 and it usually lasts till primary/sec school... Also I found that most students in top class of pri school take music lessons..
My friends send their kids for martial arts, swimming, ballet.. but some of them stop after 1 year as they don't see a need for long term lessons. E.g. once u know the basics of swimming, breathing, life-saving it is sufficient... don't need you to be the next joseph schooling haha..