Palau Guell, Barcelona

This palace was truly amazing. It's hard to appreciate the complexity of the design until you actually walk around it! I highly recommend downloading the app and listening to the (free) audioguide, there's limited signage so the extra information really helps. There's also a choice of various length tours. When we went (a Friday) it was pretty quiet, with plenty of space to explore at our own pace, whilst listening to the guide. The views from the rooftop is amazing, and can see plenty of Barcelona landmarks. Finally, it's super affordable (10-12 euro/ea), and should be a part of any visit.

While many Gaudi buildings are extraordinary from the outside, this one is a bit more 'normal'. Which is probably why it cheaper to get in than others. But while inside, it's amazing and worth the time. He really is everywhere in this beautiful home, and it's lovely to see. Make sure to go on a nicer day out, as the terrace is a special surprise but closes when it rains.