There are many activities that can be used to cover the topics of percent composition, empirical and molecular formulas and hydrates. Percent composition of sugar in a piece of bubble gum is an easy activity to begin the unit with.
Whenever possible, I try to begin a topic with something my students are familar with. For the introduction of Percent Composition in my general chemistry course, I brought in bags of Oreo cookies. Seeing the bags upon entering class was a great attention getter. If you are looking for ways to add more inquiry to your chemistry course, this a an example of how you can experiment with giving up a little control. Try it and see how it goes.