The other week, I stumbled upon the fact that loading my 16-25 Railcard onto my Oyster card gave me 33% off travel during off-peak times. I am mainly travelling during rush hour (yay sweaty tube times!) the discount is definitely useful for travel around the weekend. To activate the discount, register your oyster card at any Underground Station and take your Railcard with you. Super simple and does save some pennies! I think the tube is a great system, but travelling around definitely adds up.