In “Scramble Eggs”, the whole reason the Nakano family were there in the first place was because Miku “Won” the pair of tickets to their grandpa's inn. But that was not the case as we find out later in the arc that Miku did not actually win the “Pair of hot spring tickets”, and that it was their dad who arrange the whole family trip to see their grandpa at the pretence of her winning those tickets.
But Miku DID win something and that something WAS sent to their old home which make their dad aware of the “Spend and Win” advert in the first place. What this mean is that Mr Nakano has custody of Miku’s original prize, so what could that prize it be? My guess is that Miku actual prize is the “Pair of rings” AKA Prize-B.
It is possible that she could had won other prizes that was displayed in that notice too, but I have a few reasons to believe that it was the rings that Miku won. Because for one, Fuutarou mentions it, bringing it our attention. Secondly, it’s the most prominent prize to me because when reading from right to left, it is in the direct path which our eye track to the next speech bubble. The negative space compared to the other prizes, which it has the biggest empty area, thus stood out more than the rest. It is also the only other prize with a five-quantity limit besides Prize-A. Considering that this is a romance series, the likelihood of her winning something that would lead to a plot event or is romance related is extremely high. And lastly, it all comes full circle as the ring was mention in both the first and last chapter of the arc.
The importance of this hypothetical prize that Miku won might actually be in fact THE RING that is going to be use in the future wedding. And if its true, then Miku is pretty much guarantee to be Fuutarou future bride. And since the only person who might come to realize this little detail is none other than Fuutarou himself as he overheard the talk between two fathers at the hot spring. Once realization drawn on him, he might undergo a mission to retrieve said prize and return it to the rightful owner.