Here are some ideas to investigate further!
Pendulums:
-Galileo and Chrisitaan Huygens
-Clocks and watches
-Metronomes
-Foucault pendulum
-Create and investigate linked pendulums
-Create and investigate double-decker pendulums
-Investigate stringless pendulums (using another material as the center of the mass system such as sticks, straws, paper-clip chains, wire, etc.)