I am just about to begin my 3rd year studying Computer Science at Girton but I originally applied for Maths w/Computer Science and as such I did the 'Geek Week' as it is affectionately known.
Basically its a week of introductory lectures for 5 (I think) colleges. They are mainly intended to give you a taste of what Cambridge maths is like and are quite useful even if it's a bit of a case of throwing you in at the deep end. The lecturers do set you work but it doesn't really matter how much of it you do (as I found out... =) ).
It's a good chance to get settled in before term really starts, even if Cambridge is a bit empty at that time. There will also be 'gurus' from each college, second and third years who are there to help you through the Maths and show you around. When not in lectures you will probably spend time doing things like punting and pub-ing.
Girton-wise, the Fresher's committee will also be around, probably organising some other stuff to do.
Basically, it's not as bad as it sounds.