Go fossil hunting in the stone of the oldest buildings in town

The polished limestone and marble on banks, old stations and grand shopfronts was cut from ancient sea floors — which means the walls you've walked past for years are full of actual fossils: shells, ammonite spirals, the stacked stems of sea lilies. The fancier the building, the better the stone, so head for the oldest financial-looking facade in town and start reading the slabs. The security guard will have questions; you're allowed to be looking at a wall.