I don't use CO2.... Just medium light (with good plant bulbs)... a well seasoned substrate, and careful plant choices.
You don't "have" to have CO2... but CO2 and high light tanks do expand the list of plants that you can grow easily. For a good "low tech" tank you can use:
Anubias - I use 2 different species in my 90 gallon
Java Ferns - I use a lot of Java ferns, several different species can look really good together
Amazon swords - Surprisingly Amazon swords are a very forgiving plant. I have grown massive swords under a 20 watt bulb before
Java moss - An old favorite
Crypts - Again, very forgiving, moreso then people give them credit for. I have crypts growing in my fishbowl at work under office lighting (my desk has a basic flourescent striplight under the cabinet)
Hygro. - Many species of Hygro grow so fast you can grow them anywhere
Hornwort - I have grown this easily in the past, but its so hardy it simply took over!
Cabomba - Pretty hardy, I've done well with it at medium-high light no CO2