We do this as G and T outreach. What you will find is that a number of local feeder primaries wil have been contacted and will have been asked to send G and T children along. We have a class of between 25-30 attending (so it isn't a tutorial style houthousing! )
I can't comment on exacly what woulld happen in a maths set up, I do know that my maths oppo (I have them one we she the next) does a lot of problem solving and group work with them.
I get them for science and my brief was to extend them and that I could do whatever I liked with them.
My aims are to strech them, but I have no interest in straigtforward acceleration of subject knowledge. The first two weeks we worked on exploring the scientific method by making and testing Roman glue vs pritt stick glue. They were in groups (group work v important for these kids I feel), and had to design their own experiment to test the strenth of the glue, using their own failures to build upon and improve their thinking and problem solving skills.
Next lesson we will be analysing the results (they will have to decide what data they will be presenting and why, how to present the data, which graphs to use and why, and they can also make me some nice Wikispace entries, and add graphics etc, which I thing they will enjoy.
Death to extension work sheets is my watch word!
The next thing we will look at is some basic physics of energy exchanges, and I'll be getting them to make some Rube Goldberg machines, so they can explore potential and kinetic energy and look into efficiency.
As an asside, they are intereting kids to work with. Bright but not genuses, and many of them have to learn how to follow instructions properly if they are ever to make good scientists!
I know this doesn't explain what they do in maths but I thought it would give you some idea of the sorts of things we do with them.