Is it made of plastic? It looks so from the picture.
You could do like somewhatshocked mentioned in number 1. I would put a sponge filter in the middle and surround it with some stem plants so that the bubbles would look like the flask were boiling or something.
I think it would be hard to fit certain hard scapes into the opening, but it could still be managed.
Opae ula could work in there as well with a little lava rock hard scape.