A cave for my fish, it has two chambers for smaller fish to hide in and avoid larger or more aggressive ones.

Print took about 2 hours. Relatively simple. If it is going to be used for fish say 2+ inches, then you can snap off the smaller support as shown in one of the pictures for a larger opening as I did. Also, you may want to use a very sparse infill so that you can poke them later and release the air, otherwise it will float.