I don't really agree on the whole 'ticking a box' system associated with UCAS apps. But if there is a hobby that you've really wanted to do when younger, you may as well pursue it. I've always wanted to learn an instrument and luckily found time 2 years back to learn piano. Also wanted to get into water-sports, so started kayaking too. Just make sure you aren't doing anything because you feel you have to for the sake of your medical app.