Oddly enough, your summer vacation is actually winter in Cusco. So you're wondering what it feels like. Well everyone says it's cold. What kind of cold?
I'm from Ontario, Canada. For those of you who think that means my standard is super cold and we live in igloos.... we don't. Our winters feels similar to the winters in New York.
I can tell you June/July in Cusco feels like October-November. It is cold enough for a padded jacket. What I mean by that is not a winter jacket, but a thinner winterish jacket. Not a spring or fall jacket or wind breaker, but a little bit thicker.
You'll feel pretty cold walking around, but it's not like your cheeks or hands get cold.
Your hotel blankets at night will be cold to the touch, so it is better to have socks and maybe a hoodie and sweat pants to sleep in.
It is best to book a hotel that has a heater if you want to get good sleep. Add to the altitude sickness symptom of sleeplessness of being cold at night - its not a fun mix. Most hotels in Cusco and the Sacred Valley don't have heated rooms, but they may have portable plug-in heaters they may give you for free or rent to you for about 10 soles. So definitely check that out.
Hope that gives you an idea of what to pack and what to wear!
TIPS:
-pack a Fall season jacket
-pack socks
-pack sweat pants to sleep in
-pack a hoodie
-before booking in June/July, ask you hotel if they have portable heaters
