Puffers. Dwarf puffers would be nice in that size. You could keep a figure 8 as I believe they only get to be about 2 or 3 inches. They need brackish at first and later salt. I've kept quite a few puffers before and they've all been very "pet" like. We had one that would jump out of the water and take food from your hands, but it was a GSP. Octupus only live for about 2 years, but they can be taught to do tons of neat tricks if you're able to keep them alive in the first place. In particular i've heard you can put their food in a jar with a screw on lid and they'll get it out.