Peru is a comfortable country to travel if you understand two or three things from the outset. These are the ones that save the most trouble.
Transport
Long-distance buses are the national system and the classes vary enormously. The premium operators offer fully reclining seats, a toilet and onboard service; the difference is worth paying on overnight journeys. Distances are deceptive: Lima to Cusco is over twenty hours on mountain roads. For long legs it is better to fly — domestic flights are frequent and cheap if booked ahead.
In cities, use ride-hailing apps rather than flagging cars down. If you do take a street taxi, agree the fare before getting in, because they have no meters. Colectivos — shared minivans that leave when full — are how locals move between nearby towns and work well.
Money
The currency is the sol. Cards are accepted in city hotels and restaurants, but markets, local buses, community stays and smaller site entrances are cash only. Carry small notes. Check 100 and 200 sol notes, as counterfeits circulate; the street money changers in identified vests are legal in Peru, but compare rates first.
As a rough daily budget, excluding international flights: 30 to 45 US dollars backpacking, 70 to 120 mid-range, and upwards from there. The train and entrance to Machu Picchu are the single largest one-off expense of the trip.
Health
Do not drink tap water — bottled or boiled only. At altitude, eat lightly for the first few days. For the rainforest, arrange yellow fever vaccination in advance. Travel insurance with evacuation cover is particularly worthwhile if you plan to trek above 4,000 metres.
Safety
The real problem is theft, not violence. Normal big-city precautions apply: don't display a phone or camera in crowded places, wear your bag on your front on transport and in markets, and keep nothing valuable in checked luggage. Avoid walking at night in outlying parts of Lima, and don't accept drinks from strangers. Outside the cities the country is quiet.
Language
Spanish is spoken everywhere, and Quechua and Aymara are co-official in their regions. In rural highland areas many older people speak Quechua daily. Four words — allillanchu, sulpayki — open a great many doors.