I love this thread, very interesting opinions, as I can't wait to foist a musical instrument on my poor unsuspecting DS!
Personally, my son is going to learn the trumpet and / or cornet (I understand it's quite easy to switch between the two) and this gives lot of different types of ensembles to play in - orchestra, wind band, brass band, jazz, etc etc. Whereas the violin, for example, is so limiting!! And since up north is the home of the brass band, he'll be able to take part in stuff outside the school and with people of all different ages (so not reliant on the quality of teaching at school.) Furthermore, he can experiment with different brass instruments as time progresses, and as the interest takes him. A cheap model can be picked up fairly, well, errm, cheaply and there's a good second hand market for them. They're also not too large to carry to school and there's some quite good gig bags around these days that I've seen youngsters carrying (a far cry to my youth when you had to lug a heavy case on the end of your arm...) SO my plan is to wait until he's slightly older, as brass teachers advise to wait until their front teeth have settled down. In the meantime, we listen to lots of different kinds of music and we play with lots of toy percussion instruments, for the sheer fun of it. And the poor child gets to listen to me practising in the house...
And yes, I do have it all planned, thank you very much. And yes, I know he'll be more interested in football.... oh well, I can dream!!!