A friend ask me this question recently on Venice and I told him. Avoid St Marks during the day unless really early or after 5pm. Take gondolas in evening for best experience.
There are at least six sights in Barcelona for which it is essential to buy tickets in advance unless you relish spending 1 hour or more in the ticket line, and possibly being turned away in the end because all the tickets have been sold: La Sagrada Familia, Parc Guell, Casa Mila/La Pedrera, Casa Batllo, Palau de la Musica Catalana and the Picasso Museum.
Thank you exactly what I'm looking for!
The same goes for tickets to The Alhambra in Granada,buy far in advance.
In Seville, make sure to go slow, see a Flamenco show(we loved la casa de la memoria), reserve the real alcazar tickets a few days before, This city is on my top 3 cities. Love it. :-)
In Granada, well said above, make sure to reserve the Alhambra tickets well in advance. Go to Albaycin quarter for a great view of the Alhmabra
In Barcelona, well said before, you can also visit and see a show at Palau de la musica catalana. Wonderful architecture
In Toledo, nothing of this nature. Just wlaking in this great city, visit the must sees, enjoy the view, the architecture.
In Madrid, we reserved the royal palace in advance. I don't know if you must do this before.