Here's a ppt with a reworking of an alien drawing activity I saw earlier on waygook.  I got rid of the worksheet and just had students use their notebooks.  There is also a link to a pixar video about aliens.  I use it at the end of class as a reward, but I have also tried using it as a way to get students talking at the start of class.  I had my co-teacher leave the room during the video and had the students watch the video with instructions to pay attention to how the aliens look, so we could describe them to the co-teacher later.  The co-teacher then returns and draws them on the whiteboard according to the students descriptions.  Kids love tricking the teacher into drawing some wacky beast from their collective imagination.
Also for the drawing activity, the students should draw their own alien, but remind them to try to keep it secret.  This rarely worked for me, but I simply regrouped the students later at other tables, so they were next to someone new who hadn't seen their drawing.  For the most part, they independently started the describing/drawing exercise and engaged in some good English exchanges.
Not the prettiest ppt in the world, but should be straightforward.  Hope it helps someone. Feedback greatly appreciated.
Here's the link for vid if it doesn't work on the ppt:http://www.youtube.com/watch?v=gQYQOzSMsRQ&feature=related#VIDEO