Took a stroll up to the Kastro yet? Definitely do so. And have an ice cream while at it. Unique architecture, amazing photos. Did you know it features terraces by “public street works”? When you find yourself at the square you realise you have been walking on old houses (warehouses mostly)!
Do go over at the archaeological museum too, and the seven martyrs chapel. As for the latter, if you don’t want to go down all these steps just peek at it out from the bypass pathway at Kastro. It is worth the view!
More info here: e-sifnos.com