you can use DIY CO2, with yeast, sugar and water or the more expensive but better and more effective CO2 cannister.
I'd recommend sag - its a nice plant, and grows well. nice flowers too.
Indian fern is also a really nice plant that looks amazing if you set it up well.
kauri, oak, cedar, redwood etc have barks to rival the best in the world but I highly doubt they would do well in a tank :lol:
If you have a high tank, you can try your hand at anubias hastifoila - read my post and see the pics in this section to see what it looks like. its pretty tree like.