Singing in rounds - I do a good one which goes down the alphabet like this - start on C above middle C, then go (each dash means go down a note in the scale, so EFG will be on a B)
ABCD-EFG
HIJK-LMNOP
QRST-U and V
WXY-Z
does that make sense? The rhythm is 4 crotchets on ABCD, 2 crotchets, minim on EFG
HIJK - 4 crotchets, LMNOP - 4 quavers, 1 minim
QRST - 4 crotchets, UandV - 2 crotchets, 1 minim,
WXY - 2 quavers and crotchet for 'dou-ble-u', 2 crotchets for XY, minim for Z, minim rest, back to the beginning.
I then get them to do a clap after Z, on the third beat of the bar. Then get them going in 4 parts - part 2 starts when part1 is on line 2 etc. THe clap is really cool, because when all the parts are going, it travels along the line, so they have a visual idea of what's happening as well as aural, and it can get them back into time if they've lost it.
Also Frere Jacques, London's burning are good for getting them to hold a line.