at the dome in florence (it's very completion was an engineering feat not accomplished since the time of the romans, and is considered to have ignited the renaissance. it spans a greater distance than the largest roman dome; in fact it has the greatest span of any masonry dome in the world, still--even greater than st peter's)
pisa. 'nuff said. (Capt. Kidd leans like the Pisa)
st peter's square (and basilica), vatican city
inside st peter's. btw, that tall, free-standing brown thing is 100 feet tall and solid bronze. it stands over the tomb of saint peter the apostle. and the letters wrapped around the dome are 6 ft tall. that place is gigantic.