When I said "collectibles" I didn't mean Beanie Babies or the like. I meant things like rare coins.
Coins USED to be a collectible that NEVER went down in price/value ( as all other collectables do, due to fluctuations in taste ), until late 1989. Numismatics was forever changed thereafter; yet, it is a good hobby and somewhat of a good investment. But you REALLY have to know what you're doing and find a reliable and HONEST dealer.
Because of fluctuating tastes, ART ( paintings, lithos, sculpture ) and object d'art are fun to own, pretty good investment, but some things, no matter how old or new, can wind up losing even their original value.
Books ( FIRST EDITIONS ) are wonderful collectibles and don't lose their value, unless one doesn't take care of them.
Musical instruments are only for the MOST serious and knowledgeable collector!
Toys and now I'm not just talking about true collectibles/antiques, because many people are now collecting things less than 100 years old, but NOT anything like and including Beanie Babies, have skyrocketed in price and value. From American Flyer to Lionel to Marklin....the prices of rare, MINT trains, accssories, and their original boxes are in great demand. Stieff teddy bears fall into this category as well. But true antique toys are also great investments, but also only when their condition is very good and ALL of the original pieces are there.
OTOH, none of these things are easily, nor quickly convertible to hard cash.
There are people who collect barbed wire, beer bottles, and you name it. That doesn't mean that they are good investments.
As with homes, most collectors collect because they love the things they collect and while they might see them as investments, that usually isn't WHY they collect.