Oh yes, I've got this on the 2nd of May! My favourite is the rhythm stimulus, and like CPPF, I usually use the same kind of (bland, plain and very unimaginative) melody, which I've got up my belt. I just vary it on the day.
In terms of structure, I usually have A, B, A. each section is 8 bars long. The first and third lines of each section is the same, and the 2nd and 4th lines are similar, but the last note of the 2nd line ends in the dominant key; for the last line I end on the tonic. Then for section B, I always modulate (this gets you marks!!), and have the same structure, in a abab kind of pattern (if I have time I make the 4th line a little more impressive!). I have a link (e.g. if going from G to C maj, I put a F natural in somewhere), and go back to my section A.
That sounds so complicated in writing, but it's not that frightening! I was terrified of it when I was first introduced to it in Year 10, but now I've grown more used to it. Try not to worry too much, and if you make a mistake/forget/run out of time, just improvise! No one will know.
That being said, I know I'm going to be in a major frenzy on the 1st of May, so I'd probably be making a similar thread in a month's time...