Milkshakes are hard to make interesting. The recipe has been settled for a century, every shop's version looks like every other shop's version, and there is a limit to how many times you can put a different sauce on top and call it new.
The one part of a milkshake nobody thinks about is the straw, which is exactly why it is the part worth changing.
What happens in the cup
Color changing paper straws shift from one color to another as they chill in cold drinks, in two color pairs. The change happens after the straw goes into the drink, which means the customer sees it happen rather than being handed something that already looks different.
That timing is the whole effect. A pre-colored straw is decoration. A straw that changes while you are looking at it is a small event, and small events get talked about.
Serve it so the change is visible
Do not put the straw in behind the counter. Hand the drink over with the straw alongside it, or let the customer put it in themselves.
If the straw goes in during preparation, the shift happens while the drink is still on your side of the counter and the customer receives something that has already finished changing. They will assume it was always that color. Two seconds of restraint at the handover is the difference between a feature and a wasted case.
Where it works and where it does not
It needs cold, so milkshakes, iced coffee, sodas and floats are all in scope. A hot drink obviously is not, and a drink that has been sitting on a table long enough to lose its chill will drift back, which occasionally confuses a customer who was told to watch for something.
Paper also has a working life in a long drink, which is worth knowing before you commit. For a shake at a table this is a non-issue. For a large iced drink carried around for an afternoon, we would point you at a lid instead, for the reasons in flip top lids for drinks that travel.
Running it with the spoons
If you already stock the color changing spoons, the straws extend the same idea to the drinks side, and a customer ordering both gets the effect twice in one visit without you adding anything to the menu.
Two color pairs are available, and every pair is the same straw at the same case price. Matching the straw pair to your spoon pair makes the counter look deliberate rather than accidental, and the reasoning behind choosing pairs at all is in how color changing spoons work.
Ordering
A thousand to a case, priced in line with the standard striped straws. For a shop where milkshakes are a menu item rather than the main business, that is a long season, so this is not a decision that ties up money.
The straws and both spoon sizes sit together in the color changing range.