Grab your partner.
Reach around to their back and touch their back in one place.
Have them do the same for you.
Sing a note: "aaaaaaaa" and then raise the pitch, sliding upwards.
Have your partner move their hand up your back.
Have them sing and follow their pitch with your hand on
their back.
If they sing "oooooo" move to one side and "aaaaaa" move to the
other, and do the same for them.
Work out other standards.