Make sure you have a good mechanism for keeping your kit dry, we weren't in the wet season but it rained and hard. Don't worry about taking too many snacks, you get given stuff daily. Toilet roll is always a good one. Take a good sleeping bag and pack some thermals, high altitude camping can get cold. Metal water bottles are good as they can act as hot water bottles when it is really cold but 'plats' are good whilst trekking. If you don't like things flying in your face get a face mosi net (I was so jealous of those that had the common sense to take them). Trip to the hot baths on the last day, I'd skip it if I were you, the 'hike' to get up there, in my opinion, is not worth it, better off sharing the cost of a hotel room and getting a good hot shower that way.
Day pack rules for Machu Picchu seemed a bit 'changeable'. I had a 35 ltr pack but we told that the limit for day packs for the following 1 day entry would be 20ltrs. Luckily we were allowed in with our packs but I think it is one of those 'fickle' policies and walking poles are not allowed during your day at Machu Picchu.
Final day, before train ride back to Cusco, get something to eat, you will be on the train for several hours, they serve drinks but that is about it.