Note to readers: Keep in mind that South American summer is Dec - Feb.
Peruvian gastronomy has been experiencing an unusual boom, that is not only giving Peruvians something else to be proud of, but also creating new restaurants and new dishes almost every day. However, tradition survives strongly and is the basis of all this revolution: and Peruvian tradition associates certain foods and flavors especially with summer. It does not mean that these cannot be consumed during other seasons, but they are certainly a must in summer.
And, undoubtedly, the dishes that represents summer in Lima (and in all Peruvian coast) are all seafood-based, and the star is the ceviche (ceviche, cebiche, seviche o sebiche, too). ¿And what is this? Simply, fresh raw fish fillets marinated in lime juice, ají (chili) and some onion slices, served with some boiled corn on the cob and sweet potato. But there are some variations, and you can find fish-only ceviches, or combinations of fish and seafood, crawfish, octopus,
No one knows exactly who created it, but there are historic records that state that during the Inca times the locals prepared an ealier version of this dish, with ají and tumbo, a very acid local fruit. Nowadays, many variations of ceviche are prepared, served and consumed in many Latin American countries, but everyone agrees that its first origins are in Peru. And, for Limenians, ceviche is part of their summer rituals: not only eating, but preparing it with family and friends, since for Peruvians cooking and eating are social activities to fully enjoy. 
Ceviche and Arroz con Mariscos (Seafood risotto) on the same plate. — Photo courtesy of M. Isabel Guerra
Another very Limenian flavor, despite its Andean origins, is the Chicha Morada: this is made from purple corn boiled. The cooking water turns intensely purple and retains all the corn flavors. Chilled, served with ice and some lime juice drops, Chicha Morada is the traditional summer beverage in Lima. There’s also a dessert version of this: the Mazamorra Morada, but this is served hot, so is intended for winter. [PHOTO_218800]
Another Limenian must during summer is the Causa Rellena, a cold entreé prepared from mashed yellow potato (an exquisite native type of potato) stuffed with either chichen, tuna, avocado, red onion slices, some veggies, etc. Every restaurant and almost every Limenian has its own version for stuffing it, but the base will always be yellow mashed potatoes with some Ají Amarillo (yellow chili) and a few lime juice drops. 
Causa Limeña — Photo courtesy of Francisco Jiménez
And regarding desserts, one of the most emblematic is the Arroz con Leche; this is just rice boiled with some cloves and cinnamon sticks, and mixed with milk. Actually, this is a dessert for all the year, since it can be had very hot in winter, or cold as ice-cream in summer.
Some summer delicatessen in Lima come actually from nature: fruits. Summer is the best season to enjoy mangos, grapes and very especially lúcumas, a fruit originary from Peru (in fact, the Peruvian flagship fruit). 
Lucumas — Photo courtesy of InfoMofo (Flickr).
You can simply eat the lucumas, but you can also have a delicious lucuma ice-cream, pouring some chocolate fudge, and making your gastronomic tour in Lima an unforgettable experience. Anyway, Lima and its deli flavors waits for you. Bon appetit!