The Cusco region will see highs around 70° F (21° C) this month: a full 4° F warmer than in June. This is a great month to see the Sacred Valley bathed in sunshine without too much rain. But wet season does not fall on any one set date, and if it comes early the end of October can see the rain begin in earnest. At this time of year, the rains are usually coming in the afternoon: it's better to plan your outdoors activities in the morning.
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 this month to warrant devising a way to extend the tourist season that little bit longer: the Semana Turistica de Cusco, or Cusco Tourist Week. 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.
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: it's 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.
Cusco has a whole week dedicated to tourists this month.
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.