Unused days never expire, so you can just save them for your next trip. Also, don't forget the price from two to three day is relatively minimal (only $15 for the third day), so you should buy one of those if you can afford it and know you will be back.
Also, I've seen folks sell unused days on Ebay. I would never buy these myself, because there's no way of knowing if the days really are not used, but there are plenty of others out there who do buy them ...