I don't think there's any replacement for the amount and ease of getting stardust from gym defenders before the update.
Now it's down to:
Hatching eggs (biggest return at once, but takes a while)
Catching 'mons (100 per catch plus daily streak)
Feeding berries (20 per berry fed)
I'm guessing by effort/reward ratio the daily streak is now the best source of stardust, but it's awfully limited. But since they're still tweaking the gym update I hope they increase the dust rewards for feeding. That would also be a source of motivation to make sure you have friendly gyms near you.