Oh, you crazy kids.
You don't need to be doing anything!
Do you really think all the McKinsey types really lead antartic expeditions whilst at the same time being a pro tennis champion, the local Conservative candidate, a scuba diving expert and going on a sponsored walk across Afraica every week?
No, of course they don't.
Pro tennis champion? More like they played a few times when they were 16. Local conservative candidate? Well, they read the telegraph. Sponsored walks across afraica? Well they went on safari in kenya once.
You get the idea.
For the most part, they probably just surf the net on their laptop whlist having a dump. That would be closer to the truth.
So... don't lie, but why not be a bit creative with how you describe your extracurriculars? With a bit of self-promotion, you have probably achieved a LOT more than you actually realise.
p.s. I like your honesty. It's refreshing.