I am about to start to a run a world recovering from some terrible events. There are only a couple cities that have rebuilt over the last century. there is no major empires, countries, and the cities have just become strong enough to be stable enough to survive the world. The are very few people that survived the dark times, so besides 2 cities, there are a only a couple known settlements that support the cities.  I don't want to get to complicated on income/economy, but it seems like the standard gold/silver/bronze system would not apply..  Any thoughts on how to reward players with things beyond barter/trade as rewards. 

any ideas/thoughts or reference how to run an unusual economy like that. 

the economic system is going to be reliant on barter and trade, but that doesn't mean that it's always going to be cashew nuts or something.

You will need to figure out what is scarce, what is valuable, and what will people want. For example, I imagine that utility and durability will be more highly valued than flashiness. One of the few things that will retain value is land. If your campaign involves the party being in any one place on a repeated or extended basis, you could use the lure of land and then trading services to improve it as part of the reward structure.

Personally I'd hand wave it. Fantasy currency is usually based on precious metals, so although weight and quality may vary between currencies, the basic stuff is the same. (Scarcity of goods is another matter and may change the local cost of goods, but that's a separate matter.) It would be fairly simple for a local government to set up some sort of exchange booth where your money is assayed, valued and exchanged for local currency. Of course, there will always be a fee involved, whether outright taxation or an exchange fee. But your game, your rules.

It's a question of how long ago this "terrible event" occurred.

Some form of trade currency would probably be developed within a decade or so after people started trading again.

However - items from before the "terrible event", which the newly emerging civilization cannot yet create yet, would be quite prized.

The production of some forms of armor are surprisingly quite complex - the same for quality weapons. 

And of course, magic items - even consumable ones - for which the secret to making them has been lost, would be highly prized.

Also - influence and favors ( boons if you will ) are a good reward in such a situation: "You have the right to call upon the Delian Order for aid, one time".

Also - land, titles, rights and privileges within the society.

If the overall population is small, then presumably towns are very limited in size as well. A few hundred people? As it’s fantasy, anything goes, but if you compare it to our history there would be quite a few smaller settlements for each town (even if town only means a few hundred people). Two sounds far to few. 

Presumably this (towns) means farming and perhaps fishing as sources of food. Hunting/gathering limits any population size significantly. Continuing to look at our history, this also means that a clear majority of the population (small as it is) are involved in the production of food. Especially outside the two towns. 

With such a scenario, consider if there is one key item for everyone? Food comes to mind, but any type of ready made food spoils. Grains perhaps? If you want some common denominator, a measure of grain equivalent to feeding one person for a month? 

Such an economy - again fantasy and magic aside - would have few specialized goods to offer and farming for most would be at sustainability level. Meaning a poor harvest equals starvation and bad times for all, the towns as well.

To this you can of course add the post apocalyptic leftovers from an era of plenty. Crafted or magic items (as suggested) etc. As the economy would be different, it can mean that the value of such finds fluctuate greatly. A small farming community would offer much for a magic plow, much less for a magic piece of armor. 

The challenge of any such an a setting is magic. One Wizard with Wish can turn the economy on its head. Consider the implications for such a small population if you introduce greater magic items or knowledge. 

There are a lot of games out there that has a world similar to this, as far as rebuilding society and fighting to survive with meager resources go. You can perhaps get some inspiration from those to add additional game mechanics to mirror the setting, given that you want the PCs to engage in this, that is.

It’s a myth that older or smaller societies went through a barter and trade only system. There has always been some way to track debot etc as well as some form of coinage or economies won’t work.  See https://www.goodreads.com/book/show/17307317-money for a great history of money. It was an eye opener for me and a book favorite for sure  Well worth a read!

So I think each city state eould have their own coin system, be it marked sea shells, wood sticks or metal coins. Each state would also have a centralized authority that record larger debts. Remember the coinage in any financial system is just a fraction of the real economy. Finally traders would have an internal banking system to trade coinage and dept from one region to another. 

In such an economy coins might be rare and large amounts seldom used. Transferring wealth and coinage from on state to another might be a hassle and would need bartering with traders or lords  

Thank you everyone for some thoughts. That does help as I try to process how to handle this scenario.. FYI the "bad event" did happen a couple hundred years ago, but, very few survived, so the world is very much in a "Rebuilding phase" and most of the world is still very wild