A co2 Generator that burns natural gas would be the only other really effective way to add CO2. You can use the Yeast/sugar bombs, Dry Ice, Carbonated water, etc etc etc all you want and they are not going to make much difference unless you spend alot of money or time perfecting the PPM levels. you want 1500-1600 PPM of CO2 to make it really effective, adding just a little bit from the other methods is not going to help much if at all.
The Generator once its hooked up and running will cause heat, make sure you have a way to vent it. Its also about 2x the cost of a bottle/regulator/solenoid setup.
Tell me why you don't want to use the bottles?