You could go into a dollar shop and find a really nice decorative vase or bowl (looking for width, not height) of one gallon capacity or greater. With a heater, a soil base and some really dense planting (and maybe a couple of RCS) I think that would be beautiful.
One thing I've wanted to try for a while is to get half a wine cask, such as they sell for ponds, remove some of the planks and replace them with glass. How to get it curved would be an issue, though.
One idea I saw for transforming an ordinary glass tank into something super cool: someone got a model galleon and sunk it in their tank, making the betta look like a shark circling a shipwreck. Very cool.