Club soda may contain sodium for taste. It also comes in at a low pH due to dissolved CO2 going into solution as carbonic acid. It is also not going to give a ton of CO2 for the money and waste. A block of dry ice in a cooler with a few holes in it would be better, or a yeast/flour/sugar fermenter in the room (put it on a tray; they can bubble up and overflow).
Cat shit has high salt and will kill plants.
Most people will not benefit much, if any, from atmospheric enrichment. If light is your limiting factor as it is with flouro growers, adding CO2 is not going to help.