Marvila is what Lisbon does when it runs out of fashionable neighbourhoods and starts on the warehouses. The river east of Santa Apolónia was wine cellars, grain stores and a giant army bakery; over the last decade it has filled with breweries, galleries, a startup campus and one of the city's best new libraries. It still looks half-industrial, because it half is.
Start at the Biblioteca de Marvila — a riverside palace and grain store turned reading room with a garden out the back. From there the armazéns (warehouses) of the back streets are the real walk: craft taprooms and galleries behind plain doors that don't bother announcing themselves. Push one open.
North is the heavy stuff. The Fábrica Braço de Prata — an old munitions works now given over to concerts, a bookshop and a courtyard bar — runs mostly at night, so by day expect the shell. The Hub Criativo do Beato, the army's enormous old bakery, is turning into Lisbon's tech campus; walk the brick perimeter for the sheer scale. The Convento do Beato next door gives you the medieval version of the same riverside logic.
End on the Tagus path, walking the working river back to Santa Apolónia and the metro.
