Carbon dioxide is very easy to make and quite safe. Its chemical formula is CO2, that is two oxygen atoms per carbon atom. This combination is also very stable and does not react easily.
At room temperature, carbon dioxide occurs in gaseous form. An interesting property of carbon dioxide is that it is heavier than air, thus it can be transported in an open container without worries of it escaping. Carbon dioxide can also take a solid form and is commonly known as "dry ice".
Carbon dioxide is one of the most common compounds in the Earth's atmosphere since it is needed for plants to perform photosynthesis. During photosynthesis, plants separate the oxygen atoms from the carbon atoms of carbon dioxide and use the carbon to manufacture glucose, while the freed oxygen molecules are released into the air. This process requires energy, which is taken from the sun. This process allows oxygen-based lifeforms to survive since it converts their CO2 expiration into O2 for inspiration.
To synthesize CO2, one can simply light a candle in a flask. The CO2 will gradually fill up the flask and, if the flame is lower than the top of the flask, the flame will eventually extinguish after drowning in the carbon dioxide. Once the flame has extinguished, just remove the candle and do whatever you want with the remaining carbon dioxide. The disadvantage of this method is that soot will probably remain in the flask as well.
A second method involves the mixing of vinegar with baking soda. The two compounds react quickly and produce a massive volume of pure carbon dioxide. To capture the gas, you can attach a large plastic bag to the flask containing the reactants. This is pretty safe since the bag will only pop if too much CO2 is produced.