Ooh! When it comes to 'exotic' sports societies, I'm your man
Taekwon-do is going to get you extremely fit in relatively short time period - capoeira is also lots of fun, however it all depends on how far you're planning to push yourself to get in shape. There is also the acrobatics club - which is slightly more expensive (5 quid per session) but apparently it gives you amazing flexibility and an assortment of show-off moves
Other, recommended by my friends include breakdancing society, windsurfing society and kung-fu school
Many sport clubs hold their trainings in the evening, so next morning lectures might be painful.. but as my teacher often said: "pain is good" :P