Alasdair Kay
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I'm not sure if this is a "hidden gem" or if everyone knows about it, but it was certainly quiet when we went. Surrounded by a beautiful garden with spectacular views across Genoa harbour, Castello d'Albertis was by far my favourite attraction in Genoa (and I went to the aquarium, which is also fantastic). A huge and fascinating collection of artefacts collected from around the world, with a wonderful display of modern and graphic art on the ground and first floor. The cafe was also cheap and good quality. We loved looking at the literally hundreds of oddities and learning about the man himself. A definite must-see if you're in Genoa.