Marc's car stereo takes a usb so we made up an album as such of various bits of music. We made sure that there was at least a few songs that each person would like and no track was to be skipped. It has been brilliant actually - everyone's favourites, but also some that each person really isn't keen on..

Counting motorbikes?
When I went shopping game?
There are hundreds of things you can do to break up the fighting and the monotony of the car journey.... Maybe trying to smaller trips out, rather than big things like lego land and just be together?