Officer selection boards, are, amongst other things, looking for people with initiative who can face new and unexpected challenges with confidence. They will therefore be interested in people who have evidence of getting out and pushing themselves into challenging situations.
The classic is the Duke of Edinburgh award, which at Gold level is hard work, but there are plenty of other ways of showing the board that you are made of the right stuff.
They are also keen on people who have mixed with lots of different ages/ability groups, perhaps by volunteering, for example.