I've swapped £5 in clubcard points for £15 at PayAsUGym - you can buy day passes at various gyms without having to pay for a fully fledged membership.
It's great for me cause I don't always have the time to go. I nearly signed up for a £40 per month gym but being realistic, I was gonna go twice a week max!
It worked out that I paid £1 per visit - such a bargain compared!
You can get £5 for free as well www.payasugym.com - £5 credit
here is the clubcard link for it http://www.tesco.com/clubcard/deals/product.aspx?R=2860