Good question!

For the most part, I would imagine that these towns would be primarily commercial: Seagard is a port serving the trade along the Sunset Sea, Saltpans (to a lesser extent than Maidenpool) serves trade along the Trident and the Bay of Crabs, Fairmarket is a market town with a bridge that allows one to travel from Riverrun to Seagard or the Twins and vice versa, and Lord Harroway’s Town is located at the Ruby Ford over the Green Fork and close by to the crossroads of the Kingsroad, the River road, and the High Road. 

Beyond that, we know that Ironman’s Bay is teeming with fish, so I would imagine that Seagard would have a fishing industry as well. We know that Saltpans also has a fishing industry, and as we know from the name flat salt pans (likely due to extensive shallows at the estuary where the freshwater Trident meets the saltwater Bay of Crabs) where salt is mass-produced and sold to Essos. Associated with that, Saltpans likely also has a significant salting industry, where fresh fish are preserved so that they can be stored longer and thus sold further afield.  

In terms of other industries, I would start with industries that process agricultural resources: we know that the Riverlands produces some wine, so there’s probably coopers making barrels to hold the wine; we know that the Riverlands produces a lot of cattle, so you probably have (in addition to cattle markets) butchers, cheesemakers, and tanners and leatherworkers; we know that lots of grain is grown and traded in the Riverlands, so you need millers and bakers. After that, I would guess that between all the rivers and roads, the Riverlands also has a lot of industries that are associated with transportation: in addition to merchants, teamsters, longshoremen, porters and warehouse workers, ferrymen, and sailors, you probably also have a lot of boatwrights, cartwrights, wheelwrights, and blacksmiths who manufacture and repair the boats and wagons that carry the goods on land and water.