Inverts will depend on your lighting; you will probably have to stick with shrooms and one or two hardy soft coral species. In a tank that size, if you are inclined, you might be btter off with non-photosynthetic corals (ahermaptypic? is that right?). They would require feeding, but that is more easily accomplished in a tank that size. I would just keep a small fish or two.
For filtration, I would remove any media they provide and just use the lid for flow. Just drop in a rock or two and some sand. Lastly, assuming it's acrylic I would drill a small hole at the water line and connect a small bulkhead fitting. Then I would do a gallon water change once per week or so, using a dosing pump to do the conversion over a couple of hours or even just constantly.
Tim