The Peace Bridge is the most photographed thing in Derry and earns it. Walking across early morning or at dusk, with the Guildhall on one bank and the Ebrington Barracks on the other, is a genuinely moving experience. The design by the same engineer as the Millennium Bridge in London is elegant. The S-curve of the deck means you're always slightly turning as you cross, with the Foyle widening to the north and the city walls visible to the south. Go at golden hour.
It is free, open all hours, and connects the Walled City to the Waterside with something that manages to be both useful infrastructure and a genuinely good piece of design. The Ebrington Barracks on the Waterside end has been converted into a public square and events venue. The Guildhall end drops you directly into the walled city. Cross it twice: once to get there, once to understand it.