My personal favourite experiment: producing solid iodine crystals.
Sodium Iodide and hydrochloric acid were combined. Hydrogen peroxide was used to oxidize hydroiodic acid into water and elemental iodine, with sodium chloride as spectator ions (In a fume hood!!!). The solid chunks were filtered and washed in distilled water several times, then placed in a beaker on a hotplate. The beaker was sealed with a watch glass topped with ice. The iodine slush sublimated into a beautiful purple gas, then re-condensed into flaky crystals on the cold watch glass. When opened, an unforgettable purple cloud hung above the beaker. I’m told that later classes noticed the room smelled ‘sterile’ and was reminiscent of a hospital, though I never smelled anything myself.
Disclaimer: Iodine crystals are a DEA List I controlled substance under US Law 21 USC § 802. Please perform at your own risk.