I like a lot of neat visual treats like aquariums and terrariums and indoor waterfalls.
But another idea I’ve had would be more like a decorative demonstration of physics or something.
One idea was a table top (perhaps at a restaurant) which had a thin amount of liquid under glass and would show some kind of flow visualization. Flow visualizations are kinda neat (here’s a hexagon flow), and people would enjoy looking at them. More ideas could come from Schlieren Optics, and here’s something trippy you can do with high frequency strobe lights to demonstrate the acceleration of gravity or make neat patterns.
I’m sure I could find tons of other things besides flowing liquids, such as stuff with magnets, unusual gear systems, and instead of simple things, they could be more complex and meaningful. But you get the idea.