What you bring in your luggage will depend on where you are going and the kind of vacation you are planning. Altitude has a marked effect on climate. Normally, the higher you go, the colder it gets. If you are on the Western side of the mountains it tends to be pretty dry. If you are on the eastern side of the Andes, it can be pretty wet. Seasons also determine what kind of gear you need. Remember that there is a seasonal inversion between the northern and the southern hemispheres. That is, when it is summer in the north it is winter in the south.
Peru is very close to the Equatorial line. Sunshine is more intense so bring a hat, sunglasses and sunscreen lotion.
Bring a small foldable pouch, duffle bag, daypack or backpack to carry your valuables, a water bottle to keep hydrated and any extras like a light coat to be prepared for temperature changes.
The coast is sunny and warm in summertime (from mid-November to late March). If you plan on spending time at the beaches remember to bring proper beachwear: swimsuit or shorts, sandals for walking in the hot sand, sunscreen, sunglasses, hat, towel. It can be overcast or even foggy in winter (from April to mid-November). So, bring a light jacket if you are coming in the winter.
The Andes highlands have warm days and cold nights. Dress in layers so that you can adjust your body temperature depending on local conditions and time of day. There can even be a big difference on how you feel depending on whether you stand in the shade or in full sunlight. Excellent quality alpaca and llama wool products are available in the region and are great for staying warm at night. Visitors often choose to leave their warm clothes at home so as to have more space in their luggage to carry the warm clothes they purchase in Peru.
Daytime in the Andes can be extremely sunny and bright so bring sunglasses, a hat and some sunscreen. Chap stick or lip moisturizer is also useful since the air is dry.
The rainforest is hot, wet and humid. If you plan to travel in the Amazon make sure you bring a folding umbrella or a light, breathable raincoat, some waterproof or quick drying footwear, and long-sleeved t-shirt and pants. Quick drying breathable fabrics are best. Some insect repellent is OK, though you can purchase that in Peru.
Adventure hikes like the Inca Trail require proper footwear. Sturdy tennis shoes with good ankle support or light hiking boots. Heavier hiking boots for the more extreme hikes.