I suspect some of the underwhelmingnesses listed are down to a mismatch between expectations, time of year & length of stay (as well as eg turning left instead of right when you depart from the leaning tower of Pisa
)
We loved Copenhagen. Only had 1 short day there, a bit longer would be good. We were lucky with the weather, & saw a lot of it on a boat ride, which IME is always a great idea with limited time if it’s an option. Agree about the tiny little mermaid (but she actually looks a bit bigger from the water!)
Also loved Amsterdam. Had 2 nights/3 days in a cute hotel a short walk from the Rijksmuseum. Prebooked tickets for Van Gogh & Anne Frank. Did hop-on/off canal boat trips with guide. Weather again was fab. Walked miles along canals.
For Spanish cities try to avoid June-August. We visited Cordoba in early May, in a hotel right by the Mezquite. There’s loads to see. Granada in late October & in May - October was hotter - the Alhambra is a sanctuary on a hot day though. Lots of shade & pools & fountains. (What we also got at the end of October was the first snowfall in the Sierra Nevada - beach in the morning, snowball fights in the afternoon!)
Had 3 days in Paris during a heatwave in early April. (I’ve been pretty lucky with short break weather, thinking about it!) Boat trip was again a brilliant way of seeing some parts. We stayed in a small hotel (5 flights of stairs
) between Montmartre & Place Pigalle - wildly contrasting residents & a nice choice of metro stops. Fabulous.
Also not underwhelmed by Corfu, Crete & New York (lots more boat rides) & southern Spain - even places like Torremolinos have good points (lovely clean sandy beaches with sardine restaurants by the water!)
All the Tuscan towns I’ve visited recently have been interesting & picturesque - even Pisa. (We did swerve Florence - I last went 40 years ago, it was less busy then & I didn’t want to spoil the nice memories.) Siena, Lucca & San Gimignano were favourites.
Bath, Edinburgh, York, Canterbury, Newcastle, all lovely.
And the Cotswolds 