2) Foreign currency.
Because most nations have their own currency, and because it is valued according to the assets of that particular country, you can invest through buying and selling foreign money. You might, for example, buy the Japanese Yen if you think that the Japanese economy and its currency are going to outperform your own USA dollars. Some people buy and sell currency several times each day, in fact, to take advantage of the rapid fluctuations in this rather volatile kind of investment.
Others do it in a way that is much more time, when planning their vacations. If you are going to Europe next summer, for instance, you might want to buy Euros (European dollars) now, in anticipation that they will be cheaper than they are going to be next year.