I attended a training class and a science coach shared an activity that he does with his students to help them differentiate between observations, inferences, and predictions.
He puts a picture on the interactive white board as a warm up (he gets the pictures from a variety of sources but uses National Geographic's Picture of the Day a lot). The picture above is from the National Geographic site.
He has the students make five observations. Then he makes the students make five inferences. Finally he has the students make five predictions.
He does this every day and it really drives home the difference between those three key inquiry vocabulary terms.
I've done this activity with both my sixth and fourth grade science classes and the students really got into it and became proficient at telling me the difference between those terms.