Well, there is actual forex investing (speculating). For instance, the value of the USD versus the Argentine Peso (ARS) is something I track (my girlfriend lives there and long term I would consider moving there, current plan is for her to move here). The USD to ARS conversion is increasingly favorable for the USD, the buying power of ARS hasn't dropped significantly (in country) but the USD is worth more ARS now than 1.5 years ago (1USD:15ARS then versus 1USD:18ARS today). Someone who had currency in ARS at the time, and converted it to the USD, could see significant returns on that investment (assuming reasonable exchange fees). Almost 20% gains, that matches the US stock increases over this past year.
It is speculation, however, as you generally don't have the same sort of insight into currency movements as you will into stock movements and commodities in general. Someone watching the Brexit vote in summer 2016 would have made a decent return on ARS had they moved it to the USD before the vote and back after (changed from 1USD:13ARS to 1USD:15ARS practically overnight). But that is speculation and requires watching global and national scale events to predict.