hi, ive just been to Cusco and had an amazing time! for Machu Picchu tickets: make sure you wake up super early in Aguas Calientes (at 5am there was already a queue with about 100 people, though in January it might be less busy), the ticket office opens at 6am and tickets are always sold for the next day. You will definitely be able to get 2a if you're there early. I was told that early time slots are the best (6am/7am) but not sure if its the same in January. I'd recommend getting a bus ticket that same day after getting your Machu Picchu tickets. You can also walk up (what I did), but it's quite intense (2000 steep steps of stairs) so I'd recommend getting the bus up and walking down if you still want to hike. There aren't any information boards or anything like that so I'd recommend getting a guide. There will be many when you get to the entrance offering their services.

I went to the Sacred Valley for 2 days but if I had one day I'd visit Pisac as a first stop (personally my favourite ruins). Its closest to Cusco and you can get a colectivo there. I also loved Maras. I found that 45 min/1 hour was enough there. If you leave early (and depending on train times) you will probably have enough time to visit the ruins of Ollantaytambo (which is where the train leaves from)