Titanium disks, 99%
Titanium is a light, hard metal that is fairly inert. The chemistry of this element is best explored by dissolving titanium powder in acid rather than starting from one of its compounds as those tend to be either inert (oxide), or very corrosive (tetra chloride).
Most of the time titanium is in the +4 oxidation state, which, having a d0 configuration, is not overly exiting. Titanium +3 can be obtained and there is a surprising amount of colourful complexes that can be produced in this oxidation state.
This site uses the following titanium materials for experimentation:
Titanium metal powder