Production. Farm them up for bio farm + whip, workshop them in state property for 8-10 good tiles (and thus >30 base hammers), or do a combination if you're not in state property so you can feed the shops. Spread corporations and other per city bonuses like AP religious buildings (or those boosted by sankore/spiral) to make these cities fairly impressive.
Commerce takes to long to set up w/o the food, and GPP in a city without strong food tiles doesn't make as much sense as just getting a respectable amount of hammers for a unit crank (or later on wealth builder). Forge/factory/power on 30 base hammers can build an infantry every 3 turns on normal. Other things might take slightly longer, but as you can see this pays back the settler hammer investment rapidly and is pretty flexible. If it's a coastal city, throw in drydocks and you have decent naval help too.
These seemingly weaker cities might even help with a space ship, since you can put parts that you've researched there relatively early and save more expensive/later ones for bigger hammer cities.