I do this activity as an introduction to the nature of science.  An object (not easily guessable) is put into a paper bag.  The job of the class is to figure out what is in the bag. At first, the students are simply shown the bag (sense of sight). Discussion (hypotheses) ensues on what could be in the bag.  I then walk around and shake the bag so students can hear what's in it (hearing). This results in more discussion, with some previous ideas being discarded. The bag is then passed around and students can feel (but not open!) the bag and also try to smell it.

In this activity, Introductory Chemistry students are given two possible structures of sulfur dioxide, and based on an assessment of given vibrational modes, they determine which of the modes are IR active (and thus, whether the molecule is a greenhouse gas).  They compare their assessment to experimental IR absorption peaks, and the students decide which structure is valid.

This in-class activity is designed to give general chemistry students practice with drawing Lewis structures. Small groups of 3-5 students compete for points by creating hypothetical molecules that meet criteria (numbers of elements and atoms) assigned by the professor. Beginning with simple molecules, the basic challenge format calls for increasingly complex criteria in successive rounds of competition. One optional variation also allows student groups to challenge each other for bonus points.