Go to Apple Market and kick the can into the bakery. The woman behind the counter will give you 1000 Zenny. Enter any shop and exit. The can will reappear. Repeat this as many times as needed. Note: This will still work when everyone leaves.

Go to the market just before the main city (the junk store is found here). At the south entrance/exit, you can find a small yellow can on the ground next to a garbage pail. Start kicking it until you put it behind the counter of the larger of the two bread stores. A "Nice shot! Bonus 1000 Zenny" message will appear, followed by Remember to recycle your empty cans". You can exit and re-enter the market and repeat this as many times as needed.

Play the beast hunter game at the Studio to get 4000 Zenny for completing grade A. Repeat this as many times as needed.

Once you have destroyed the Bruno robot in Old City, go in the gate down to the Main Gate. Destroy the fire breathing dog. Collect the Zenny with the Vacuum Arm. Exit where you entered, and repeat as many times as needed. Before that though, there are "beehives" in the Cordon Ruins, where you get the Yellow Refractor. As long as you do not destroy the "hives" themselves, they unleash a unlimited number of flying enemies.

Use a high powered gun such as the Shinning Laser in places like the Underground City, Uptown, and Downtown to get more money than with the Buster Gun.