I have had success with 2" of hydrotron expanded clay beads topped with another several inches of coco husk peat. The hydrotron wicks extra moisture back up to the substrate but allows excess water to drain as well. The substrate stays moist all the time but not wet, eventually the plants will root into everything but I still like to have a drainage layer.
Ferns are a good choice for low light but make sure you have a small species because they will grow large fast in a terrarium. Moss will not grow without lots of light, sphagnum only grows in my tank when it is within 12" of the lights, other mosses are growing on the floor about 24" from the lights.
All I have for lighting above my tank is a 23watt compact florescent Envirolite bulb, no fans and I hand mist every morning. If you can give your terrarium enough light from a window without generating too much heat inside you should be fine. What exposure were you planning on?

Your idea sounds good to me, Dahlia Guy! I assume that my 'normal' you mean a 10 gallon tank. I wouldn't add any animals. They are not just a novelty for a terrarium, but they are a serious investment in time and money. You would need to totally change your plans to accomodate the animal's needs, including substrate, specific humidity and temperature levels, etc. So unless you've always wanted a lizard or frog for a pet and are willing to create an enivironment in which it will thrive, not just survive, then forget it.

Your terrarium sounds nice and low maintenence. In addition to the ferns, you could add some african violets, especially the minis. Or try fittonias, baby parlour palms (Chamadorea elegans), english ivy (if you keep it trimmed back), or very small dracaena cuttings (this includes Lucky Bamboo). It does depend on just how low of light you wanted to put it in though. And as for your lid, the screen is useless. It will not hold any extra humidity in. Get a glass or plexi top, and you can leave part of it open to allow excess heat to escape when the sun hits it. though honestly, I haven't found this to be a problem at all, so you could just completely cover it, unless you want to put it right in a south window. I would also very the height of your landscape. Don't just make it all flat with tall plants in the back and short ones in the front. Bo-ring! Give it some slopes and valleys, and make them dramatic enough to be noticed. You could also add some interesting bits of driftwood or rocks for greater interest. If you terrarium will be very dimmly lit indeed, even a small bulb would help a lot. Otherwise all of the plants will probably lean towards the light, and you also won't be able to see them very well, especially if you have condensation on the glass (which you should, because it indicates that the humidity is nice and high). You really don't need a fan, but make sure that you only use RO or distilled or rainwater. This will avoid hard water 'stains' on the glass from misting, and will also avoid the build-up of salts and chemicals that would eventually kill all of the plants, since there is no drainage in a glass tank! =) Oh, and water your plants so that they are just moist, the way most plants like to be, before you put them in, and then you will need to keep an eye on them, because they can go bone dry without you really noticing, especially with a layer of moss on top. But they will stay evenly moist for a surprisingly long time, especially if the humidity is good, so it will be low maintenence. And when you do water, water with a light hand (obviously). Easier to add more water than take water out! =) I hope I haven't made this sound needlessly complicated. It really is quite simple to make a very nice terrarium if you just pay attention to a few details, and those 'mini environments' do look so cool! =)

I don't know about normal being a 10G sahoyaref. In my family normal means you need help carrying it. I do agree with your animal assessment though. The correct thing is to design the home for the animal according to it's needs. Anything else is no less than murder.

Well, dragonthoughts, I think you are the exception rather than the rule. =) Most people I know get 10 gallon tanks because they are the cheapest. Though yes, up to 55 gallons is also fairly common.

I agree with Sahoy in the set up for animals. You can go to www.robsviolets.com or www.blackjungle. and find out about plants and their needs (humidity, light, etc, etc) One thing: I planted most of my plants in small pots (2" and 3"), however, I just finished removing them all, because they dry a lot faster than planted directly in the substrate, mine is 2" pebbles, 1/2 charcoal, 3"of potting soil, I have a plastic screen in between the layers so the charcoal and soil won't filter to the bottom (the screen is the same as they use for window screening, but not metal,is plastic.
For dahlia_guy only, the rest already seen it....:)
Here's a pic of my terrarium after I planted it, it has growned since.
Ana :)
PS:

Here is a link that might be useful: My terrarium