How about that annoying little insurance lizard?
All kidding aside, a 10gallon would make a great tank for brilliantly colored guppies or killifish. Although small, they are still fine fish to keep. They're both active and interesting. The guppies are much easier to acquire than killis, and the really colorful varieties of guppies are also much easier to come by than the spiffy killis.
You could put a couple live plants in there with a few guppies and have a great looking tank at minimal cost.