Develop a hobby or skill, and improve at it. You should spend 1-2 hours a week doing this - at Gold level you could do 3-4 hours a fortnight
So many things you could do. Here are some of the most popular
Learn a musical instrument (Practice time can count towards your total hours, as long as your assessor is convinced you have been practicing!)
Drama and Theatre groups.
Cooking / Baking, but make sure you evidence it
IT and programming but make sure you evidence it
Art / Creatve crafts, but make sure you evidence it
There are so many more things you could do, check out this list which is by no means exhaustive!
Whilst Physical activties are skillful, they are not a Skill for the DofE Award. Don't get caught in this trap! The other one to be careful of is 'Coaching and Leadership' - unless this is leading to a recognised badge in Scouts / Cadets or a formal qualification like Young Sports Leader, is is unlikely that this can be counted, and instead should be a volunteering activity.