Okay, super, super easy:
water wisteria, marimo moss balls (though this really isn't to swim through, just more water cleaning), anachris (comes in a variety of types), floaters (like water lettuce, frogbit, salvinia, duckweed, and fairy moss (my favorite)), anubias. Those are about as low maintenance and easy as you can get from what I've experienced thus far. :)
Water wisteria can be planted in gravel or it can float, if it floats it turns into a sort of bushy/leggy/rooty thing, but some of my guys love it because they can hide at the surface in it.
Anachris can do either. You usually plant it, but I've had some that broke free and floated and did great. It looks a little strange floating because it still grows lengthwise, but does fine and stays near the surface.
floaters obviously float. Just dump them in.
anubias will need to be tied down to a rock, driftwood, a decoration, something, but you just use white cotton sewing thread or unflavored dental floss to do that.
These are all pretty low-light, I just do a 12 on/12 off cycle with two clip-on desk Lamps on either end of my 20g long tank (you could probably do with one if your tank is shorter) bent so that they're low over the water and pointing straight in to maximize the light they're putting off, and a 100-equiv-watt compact fluorescent light (just normal light fixture fluorescents, like you'd use in any lamp or light fixture in your house). All my stuff is growing well with that.
I fertilize...occasionally...when I think about it...with plain flourish (some people have issues with anachris melting with some of the other flourish products, but this on is safe), no CO2, no root tabs, nothing. Plain small diameter gravel in the bottom of the tank...2 fish and some snails (pond, apple, assassin, and ramshorn scattered throughout in various tanks) to provide natural fertz, and water changes as chemistry calls for. :)
It's about as low-tech and easy as you can get. ;)