When visiting Sevilla you absolutely must go and have breakfast at Chocolateria Virgin de Luján and have the best churros with chocolate milk ever (I am actually not even sure there is milk in that chocolate drink). Nothing touristic about the place, just plain and simple churros con chocolate with the locals. Note that they use their own produced chocolate. So, forget your diet for once and just dip that churro in the chocolate drink.
It is a little outside the city’s touristic center, but still in walking distance (±1,5 km from Puerta de Jerez), and you can also take a bus or the metro (metro station Parque de los Principes). Make sure you go to the right location (Calle Virgen de Luján, 41) because they moved a few years ago and some maps and guides may still point you to their old location. It is quite close to Triana, so start the day with a nice treat by having breakfast at Chocolateria Virgen de Luján and then take the opportunity to visit the Triana neighbourhood.