Weather
The Cusco region will see highs around 70° F (21° C) this month: a full 4° F warmer on average than in June. This is a great month to see the Sacred Valley bathed in sunshine without too much rain. Bear in mind that partial cloud cover, such as that around the Sacred Valley this month, can often enhance the views for those Inca Trail snaps.
But wet season does not fall on any one set date, and if it comes early the end of October can see the rain already coming down in great quantities. At this time of year, the rains are usually coming in the afternoon: it's better to plan your outdoors activities in the morning. At all times this month out on the region's multi-day treks, be prepared for showers and downpours, with appropriate wet-weather gear.
Underfoot, the going is decent but getting boggier as the rains begins to arrive more and more frequently.
Crowds & Costs
Cusco will still seem busy compared to the vast majority of South American tourist destinations this month. But authorities have clearly felt that there is enough of a drop in visitors in October to warrant devising a way to extend the tourist season that little bit longer: the Semana Turistica de Cusco, or Cusco Tourist Week. See here for ideas on how to spend 24 hours in Cusco. You are perhaps slightly more likely to be pounced upon for your custom at major tourist destinations in the Sacred Valley this month, because there are less international visitors around.
It is worth shopping about for a deal on a hotel or tour. There are many more rooms available, and some hotels may give you a discount or upgrade you to a better category of room for no extra charge.
Chat with a local specialist who can help organize your trip.
Where to Go
Cusco, gateway to the Sacred Valley and Machu Picchu, deserves some of your time this month, as the Cusco Tourist Week puts on a wide range of events aimed at foreign tourists. The Inca Trail and Machu Picchu still remain quite crowded, so the temptation remains to go off exploring some of the less-visited places and less-busy treks in the Sacred Valley. Come the heavier rain next month, outdoors destinations will seem much less appealing!
What to Do
It is the last month of decent trekking weather, but lots of visitors are aware of the advantages of this time of year and big attractions like Machu Picchu remain busy. The Inca Trail, too, will still be quite busy, albeit much less traipsed than during the last three months. It is a great time to try one of the lesser-done alternative treks to Machu Picchu such as the Lares trek. See here for a 6-day Lares trek itinerary.
Shoulder months like October are also great for doing a trek involving jungle as well as mountains, such as the Inca Jungle Trek, which involves cycling, zip-lining, and rafting as a 4-day adventure tour build-up to Machu Picchu. There are fewer crowds, enough water to have swollen the rivers a bit which makes better rafting and no total wash-outs as wet season has not properly begun. October along with November and March is the best time for this particular activity.
October Events
Semana Turistica de Cusco: This seems like a touristy crowd trap on paper, but the events put on throughout the city during Cusco Tourist Week held at different times during October actually give a lot of interesting insights into what the region offers.
More Helpful Information
Peru in October
Machu Picchu in October
Ultimate Guide to the Inca Trail