When most people think of commodities, they picture complex futures contracts or a staple product like crude oil. But the term commodities can apply to a wide variety of hard assets, as collectors and antique hunters have created a commodity class all on its own. The list of alternative commodities includes those that you could not physically invest in, but rather can physically hold with the hope that the price will appreciate as time goes on.
This may sound a bit foolish, but think of someone who has Babe Ruth's rookie card in mint condition, or a grand piano from the 1920's; those assets have skyrocketed in value, and there are hundreds of people who would pay a premium for these rare items. These commodities are extremely illiquid, but also come with relatively low risk; there can be no futures speculation or market makers for these assets.
As far as investing is concerned, you won't find much in the way of direct stocks or other securities; it will likely come down to you physically holding and storing these assets yourself, but for most people, that is half of the fun.