Hobbies are HereditaryYou cannot pick someone else's hobby, but you can infect them with one! The best way to give your child a hobby is to have one yourself. Your child may pick your hobby if they are interested in closer bonding with a parent, or they may pick a different hobby if they are looking for a way to express their own individuality. Either way, your example will tell your child it is OK to have a hobby of their own.
Support Your Child's Hobby
That doesn't mean just giving them hobby money, and it doesn't necessarily require you to actively participate in the actual hobby. But it might mean building shelves to display their finished car models, hanging airplane models from the ceiling with fishing line, listening while they describe an esoteric painting trick, or driving them to an early morning Sunday tournament at the local store.
Hobbies are Not ChoresIt's not a hobby if you have to make them do it. "You're going to play Warhammer and like it!" is not really going to help.
Also, let your child continue to choose their hobbies. Helping your child schedule their time to enable the hobby is one thing, being a strict taskmaster about a leisure time activity is something completely different.