A betta would be good in a five gallon. They have tons of personality.
As for any sort of indoor pond, a half whiskey barrel with a liner, or you could also buy the fake plastic ones, is one option. It would be a pond of about 20-25 gallons. You will need a pump and other accessories. Also, you could just use the liner and make a pond out of any sort of reservoir. In a pond setting, you will need a lot more water, so this wouldn't be adequate. Not for a couple full grown goldfish.
When you are thinking of an indoor pond, exactly what are you thinking of? I was thinking small, but you can build bigger ones if you have the space and money.