Well, if you did want something living, that would still be a nice size for shrimp or a snail. Maybe check out your pet store's selection of those (they come in different colors) and choose a plant, stone, glass, etc. based on the shrimp or snail.

Umm, maybe do a color scheme, or mini bonzai tree scheme, or you could grow bamboo. The 3g would comfortable fit a pretty
betta.
Wait...I'm thinking of the pre-made ready-to-go 1 or 3 gallon bowls. Do you just have a literal bowl? In that case, Bamboo may be your best option. It grows well and it's very pretty.