The only thing i thought of at the moment is Oil on Water photography...i was just doing it for fun but my friend who is a photo teacher asked me about it so i think she may have used it. Here's a couple of my examples:
IMGP1554-1 by
L. Trozzo, on Flickr
IMGP1642 by
L. Trozzo, on Flickr
Basically its oil on water with a background. Here's the tutorial i followed as a guideline:
Oil on Water photography
Edit:
Is is something you want them to shoot in class or out of class?
I dunno if this is really appropriate but i had a good time with shooting smoke:
Untitled by
L. Trozzo, on Flickr
Smoke by
L. Trozzo, on Flickr
Or they can experiment with the shutter...?
Untitled by
L. Trozzo, on Flickr
Untitled by
L. Trozzo, on Flickr
Okay last thing, are you using photoshop? Composites are cool too
Composite by
L. Trozzo, on Flickr