Coins, Stamps and other Collectibles.
There is really not much to say about this topic other than if you do not know what you are doing then do not do it. Rare coins, stamps and other collectibles can be worth a fortune but, more often than not, they are not even worth what you pay for them. Collect them because you like them and not because you think they will make money. In many cases coins are sold as being "limited mintage" and it is implied that because of this they will be valuable. Unfortunately two things have to happen before anything becomes valuable, the first one is that there must not be many of them and the second thing is that people must want them. You may have the only coin of its kind in the world but unless someone wants it badly enough you will not be able to sell it for very much.
If you want to try a bit of a crap shoot then you can buy packets of old coins on Amazon. Just buy them and keep them in the hope that in 20 years time you will have a coin that is considered to be rare or maybe one with a key date that you can sell for a lot of money. If you have a spare $20 or so then it might be worth taking a gamble.