I think it's a cute idea!
Don't paint it - there will be dark green algae all over the insides soon enough.
A fountain made out of a water spigot would be cute. You can buy these in catalogues, or other places. I would be afraid of leaks, though, so would want it inside the tub. Alternately, you could use a small half-whiskey barrel, and let water spill over the edge down into the tub. You don't need a lot of sound - just a little is more soothing than a big crash.
You can have marginal plants by standing their pots on top of an strawberry pot or a milk crate, so they are mostly out of water. Floating or underwater plants, too (hornwort, anachras, water lettuce, water hyacinth, duckweed).
A few goldfish (like, 3) would be fine, even without filtration or air. The plants will filter the water a little, and also add air.
You just have to figure out how to stop the bottom drain, so it doesn't leak. Say, if you left is useable, your friend would be able to drain and clean the tub/pond easily.
