Retiro Park and Alcalá Gate and a walk down Paseo del Prado/Paseo de Recoletos to Fuente de Neptuno, then to Plaza de Cibeles.
If you’re a Botero fan, his Lady with Mirror sculpture is at Cristóbal Colón plaza further down that avenue. There’s another of his sculptures outside the airport terminal. At the same plaza is the Lucky Frog sculpture.
The Plaza Mayor and Puerta del Sol area is good for people watching. Beware of pickpockets.
The Royal Palace over to Temple de Debod, then back to Plaza de España with the statues of Don Quixote and Sancho Panza, then to the Gran Vía for building architecture.
For lesser sites, the Puente de Toledo and Puerta de Toledo make for a short stop. En route there’s the Catedral de Santa María la Real de la Almudena with its modern interior.
If you’re a Francisco Goya fan, an off the beaten path site is Ermita de San Antonio de la Florida. It has a few of his paintings and his tomb. It’s free to enter and doesn’t take long to see.
With your very limited time, I wouldn’t spend a chunk of it visiting museums. However, if you found yourself with a little time in the evening, consider the free hours at the Prado and Reina Sofía. You could get a good feel for one or the other without sacrificing site seeing time.
I too recommend the mentioned places for churros and chocolate. Both are within a couple minutes walk of the Puerta del Sol and Plaza Mayor. Mercado San Miguel off the Plaza Mayor is a popular food venue and it’s always very busy.
I’ve never purchased a bus pass since every place I mentioned is walkable. Not that they are close together, but walkable.
