I prefer putting together a variety of cargos based on what the departure planet may have for transport, and then let the players determine where they are going next.
Say that the departure planet is Poroxlo for a Jump-1 Free Trader, with possible ports of call either Rhylanor or Jae Tellona. Rhylanor is a high population planet with a very thin and tainted atmosphere, while Jae Tellona is a Desert World. Porozlo is basically an Terra-normal planet, albeit with a high population and is a Balkanized world. Agricultural products would probably command a good price on either Rhylanor or Jae Tellona, with any form of sea food being exceptionally profitable on Jae Tellona. Wood products may also find a good market, while slightly higher Tech Level items would be good for Jae Tellona. Possible small shipments for the Navy bases might also be available, and may have higher than normal shipment payments if a priority cargo. Then, as Porozlo is a Balkanized World, one of the other governments may maintain a small starport under its own control for its own subsidized trading purposes, and be willing to make a willing Free Trader a highly profitable ship.
There are all sorts of possibilities for cargo with just those three planets to work with.