A LARGE number Jeopardy games. One for each GCSE strand - algebra 1, algebra 2, number, trig, shape and space, geometry, statistics/probability etc. Plus some more general ones for KS3.
Check out www.ibmathsresources.com for loads of dingbats to customise the challenge section.
The game works well by giving each team a whiteboard, which they have to hold the answers up on. Expect to take 40-60 mins. For some of the later IGCSE jeopardy games I've left the Challenge sections the same - you can customise these yourselves.