Machu Picchu

Historic Site
Various, but generally expensive.

An absolutely stunning site, high up in jungle covered mountains. The site itself is pretty good, but it's the setting that makes it so incredible. I would recommend buying tickets for either Huaynapicchu or Machu Picchu Montaña; the views over the ruins and the surrounding area make the effort worth it. Note that the Montaña is significantly higher than the rest of the site and it is quite a bit of work to get up there, but it really is awesome. However, there are additional ruins up Huaynapicchu, so it might be a better option if you're a completionist :) They don't seem to sell combination tickets for both, unfortunately, otherwise it would be possible to do both in a day.

Tickets can be bought online (if you're lucky), through agents in Cusco (more expensive) or at the ticket offices in Cusco or Aguas Calientes. The ticket office in Cusco is probably the best bet; it used to be very crowded, but most agents now buy tickets online and take an easy commission, so the queues are small to nonexistent. Booking in Cusco is a bit of a safer bet (especially in high season) than in Aguas Calientes. There are a few ticket options, so read about Machu Picchu tickets before buying.

Getting here (well, to Aguas Calientes) is a bit of a pain, with expensive trains being almost the only option (going the long way around via Quellomayo then hiking is possible, but time consuming and tiring). You can book these online or via the Peru Rail or Inca Rail offices on Plaza de Armas in Cusco; note that most departures come and go from Ollantaytambo in the Sacred Valley (all of the Inca Rail departures), not from Cusco, and even the Cusco ones leave from a fair way out of town.

Getting up to the site is also pricey. Return tickets by bus currently cost US$17 (one way for US$9), or you can walk up yourself (appx. 2 hours, apparently). Bus departures are frequent, although queues can form during peak times.