Munnar and Alleppey both are special in their own category. If you looking for a short time holiday vacation then you should go for a Munnar.  You can also get some thrilling adventures experience in Munnar. And Munnar known for the amazing hill station. On the other hand,  Alleppy is situated at an elevation of 11 meters,  so it's not considered as a hill station.  Alleppy is known for a honeymoon destination. You can also visit here on your holiday trip.  The most popular thing in Alleppey in Houseboat riding in the backwater. So, first of all, know your interest then go for that.