Dear Karyn963,
What fun! 39 gallons to play with! For north-window-character light/part shade and good humidity IâÂÂve had good luck with
1. miniature begonia for some height; there are also vining ones (check out the choices at violetbarn.com and blackjungleterrariumsupply.com). The one I have is Begonia dregei âÂÂmaple-leaved begoniaâÂÂ: it can be pruned as needed and grown as a pseudo-bonsai to provide a small tree for your terrarium landscape). When allowed to grow it makes lots of white flowers.

2. mosses! Definitely. Go for the carpeting forms (pleurocarps) that grow on the ground in shady places. Literally. Get several kinds from your backyard.
3. ferns! Definitely. With the size of your tank youâÂÂll have lots of options. The ones IâÂÂve used in much smaller terraria than you have are Lemmaphyllum microphyllum (bean fern; I got both forms of it), Selaginella kraussiana âÂÂBrowniiâ (pincushion spikemoss), Asplenium x ebenoides (dragon tail fern), and Microgramma heterophylla. Ones on my to-try list are Nephrolepsis exaltata âÂÂElzevirâ and button fern (Nephrolepsis Dufii or the different version available at glasshouseworks.com). Also check out rareferns.com, blackjungleterrariumsupply.com, and glasshouseworks.com for others.
4. micro-mini Sinningia pusilla. Should bloom for you: totally sweet and amazingly robust.

Check out the many other terrarium plants offered by the sites IâÂÂve mentioned above and note ones you like that have similar light and water needs to the other plants you have. Pay attention also to the mature size to be sure they wonâÂÂt outgrow your space.

Have fun!