Sponge filters would like work well for you. You can vary the size of the sponge, and hide it behind other decorations. You will have bubbles from the air line, but this typically isn't a huge issue. They are easy to clean, simply remove the sponge, swish out in clean, dechlorinated water, and you're good to go.
I have an AC 150 on my 5, and it is wonderful, though it doesn't create a bit more current than other small filters. I like that, since it traps more waste in the filter than the slower flow ones do. Since the AC 150 hangs on the back, you only have a single tube inside that tank, as well. I think many people use the AC-Mini instead, but I like the 150 much better.
There are also aZoo Palm filters--never tried one, seem to work decent, but they are tiny--I wouldn't rely on one for a 5.