I think the toast one's something to do with the average height of a table and the rotational speed of the falling toast (as it starts buttered side up).
As it's a biology class, how about "you're more likely to catch a cold in cold weather."
Find someone with a cold - get them to sneeze on some petri dishes; refrigerate some, incubate others, and controls at room temperature. See what conditions the virus thrives in the most.
Nice and cheap experiment that basically involves waiting
