Shallow rectangular pools, like the one in front of the Washington monument in DC, designed to be placed in a garden. We could even put the water lily clusters there, if allowed, This could basically just use the same tech as the hot tubs, and because the purpose is just decorative, remain shallow enough so its just one stone height above the ground. (More than one stone height I don't think would look OK.) Probably enough room to fit some koi, too. You could also get fancier and make some to make some to put in front of estates and castles, with platforms for placeable sculptures and fountains. You could also do a round one like in that dungeon room.