If it were me I would plant heavily with live plants in pots and mist it several times a day. Substrate harbors bacteria and other problems. I'm not a Jackson's keeper but I think most would agree with the no substrate method.
I have a potted Umbrella Plant, a hanging Pothos, and some tillansias in my Jax's cage. He likes to sleep on the tallest Umbrella plant branch . I just have paper towels that I change daily for the bottom of the cage. For humidity I have a reptifogger and I mist with a spray bottle 3-5 times a day, and a dripper that drips into the Umbrella plant.
Edit - I have also read that ficus trees are good too but I read somewhere else that they can have some negative side effects. Here is a quote about ficus: "One commonly recommended plant which should be avoided is the ficus tree. This plant is mildly toxic and excretes a white milky latex. This secretion has been connected to a number of eye infections in captive chameleons, so this plant should be avoided." and that quote is from this PDF about Jackson's: http://www.skypoint.com/members/mikefry/jacksons.pdf