As a local, I am in love with Sevilla, my hometown. And although summertime is the season when the city remains quiet, it´s also the time to visit it despite the hot weather. There are plenty of summer terraces and rooftops with fantastic city views, either for drinks or dinner. Check some of the best ones on this post (thanks to my friend Shawn) . I also recommend dining out with views at places like the Vincci La Rabida or Fontecruz hotels.
Also very recommendable are the summer nights activities at the impressive Alcazar Palace, one of the city´s landmark monuments. More info here (Spanish)
The Cathedral, Fine Arts Museum and the Archive of Indians are usually quite crowded during most of the year, especially the cathedral and Giralda tower, but July & August are perfect months to visit them and lose the crowds.
Finally, a walk by the riverside at sunset, a visit to Triana market for tapas or a stroll around the Maria Luisa park by the beautiful Plaza de España are very good options for a summer in the city. Private tours, hotel recommendation and booking and special activities can be booked through my services directly if necessary.