Real plants do indeed keep the humidity up and more importantly, more stable. Pothos is a great terrarium plant, hard to kill, grows like crazy and support the wieght of RETF. Snake plant is good as well. It's not as hard to kill as pothos, but they have VERY strong, thick leaves.

You should check out the "What does your tank look like" thread in the general section more some inspiration and pictures of the plants!