There is a post on the forums here about which kinds of plants are ok. Also read through others posts in the enclosures thread to see what they are using. I have a pothos in his enclosure and I'm trying to find a ficus tree to free range.
As for the humidity, I use a basic humidity gauge that sits at the bottom of the enclosure. I got it at home depot for $3.
really depends on your cham. many use ficus or pothos. i currently use three dracena marginatas(in one planter) for my panther, but he is still younger than a year; the dracena leaves are too long and weak for larger chams to climb on(although they do use the stalks). hibiscus is also a good one, but much tougher to keep healthy indoors(in a cage).