I make a Sugar Yeast brew at home, its pretty low maintenance and depending on the size of the reservoir, it adds about 200-500 passive co2 ppm for my plant in the grobo. Since my grobo is in a sealed grow room and also is my work office, the passive CO2 in my room is much higher than normal.
You can also add a small mushroom grow if you are concerned with the lifetime of the CO2 gas. Added CO2 gas wont add much benefit during veg so if you want to budget on resource, save the CO2 flowering stages. The added CO2 will also help growers who have issues with high temps or limited to spaces without adequate airflow.
The green pads shared by @Centervillain works well too, however they need to be replaced regularly as well, they last longer inside propagation domes. I would recommend CO2 for grobo users.
I for one am glad that the grobo team recognize the CO2 challenges the grobo may have and doing something about it like adding some kind of CO2 solution to the store. I personally feel CO2 plays a huge factor with yield and growth and is what allowed me to add so much light to my previous grow to max out yield without killing her.