Two things you need to supply:
1. How hot does it get in the terrarium and how cold does it get.
2. Do you know the footcandles at the bottom of the terrarium?
Also because of its size I believe your best bet is with bromeliads and ferns. If cool enough you could even use mosses and liverworts.

Here is a link that might be useful: dutch vivariums

1. I'm not sure, I don't have a thermometer. The top is open, but for the most part when I open the front doors I get a blast of air that feels about 65-75 depending on the time of day, and I'm pretty sure the water, which feels about 60-65 cools off the bottom of the tank quite a bit.

2. The exact lighting is, once again, unknown. It sits right in front of a window that doesn't get much direct sunlight, but the shades are usually closed anyway...so it gets some light but is pretty dark. Think...forest floor towards the end of an overcast day with a thick canopy. Pretty shady. That's the best I can do, sorry!

What ferns like damp conditions and can be mounted on logs?

Thanks a lot for the response!

If you can mount using green moss that is wrapped around the roots you can use several plants. Use the soft wire that is covered with plastic and secure snugly.
Here what I can recommend:
1. Selaginella kraussiana-put this near the bottom as it can handle deep shade and it will spread.
To learn more Click here

2. Polypodium aureum-this is only one of several epiphytic type of polypoid fern
To learn more Click here

3. Pleurothallis-this is an orchid. A small plant of which many species can handle shade.
to learn more Click here

This about all the time I can spare at the moment but here is a link to a great place that sells alot of tropicl plants. Be sure to Favorite this link:
Glasshouse Works

If you are on dial-up be patient as these pages are large!

My links (and hence typed message) apparently won't post so will shorten message. Try Anubias they grow either submerged or emersed depending on the type (great variety but hard to find many, I've had good luck with ebay or aquabid). Do not bury rizome but let rest on soil so roots get fertilizer. Can be tied to wood, weighted or onto rocks (better craggy rather then smooth so they can atach). There is a great site but can't seem to link url. Any one can email me and I will send links. Has great information (more then most sites) and super photos and shows great variety. Best Sherry

Blackjungle is a website with tons of plants for sale. They have orchids, anubias, and tons of other plants for sale
Hope this helps
~Chris

Here is a link that might be useful: Blackjungle