Citric acid + baking soda is actually a way described in a procedure in an academic journal. I want to say it was a study done in Egypt? Maybe Korea? I have to go back and look. Basically, they used a small cup containing citric acid (canning additive or lemon juice would work) inside a container. Dumped baking soda into the cup, closed the container with a small opening to let air escape while CO2 formed.
The best CO2 setup I have seen thus far is from @ncdabbler