If you like color other than green, I highly recommend Red Frill (from Australia) And Red Ludwigia. I got both for very inexpensive at Petsmart, and they are the best growing plants that I've ever introduced to my tank.
Another fun one (it takes time, so be patient) are the ever popular "betta bulbs." You essentially just drop them in the tank and let them sprout.
Most tanks will not have Co2 issues when dealing with size less than 10 or even 29 gallons. But a home made CO2 diffuser can be made with a 2 liter pop bottle, 5 dollars of brewers yeast, a package of sugar, and last you a year.