Portugal has one ski area, and the more interesting answer to winter there is everything the cold months make possible that summer does not.
Serra da Estrela
The country's only ski area, on mainland Portugal's highest point at around 1,993 metres.
Small by Alpine standards — a handful of runs — with variable snow cover and an unpredictable season. It functions as a day out rather than a ski holiday, and Portuguese families treat it as exactly that.
The natural park around it is the more substantial attraction: glacial valleys, granite boulders, and the Zêzere valley, with walking that is excellent in spring and autumn and serious in winter conditions.
The mountain villages — Manteigas, Linhares, Piódão nearby — are the reason to stay.
Nazaré, which is a winter event
October to March, when Atlantic storms drive swell into the underwater canyon offshore that focuses wave energy into the record-breaking waves the town is known for.
Watched from the Forte de São Miguel Arcanjo headland, and the surfing itself is for a tiny number of tow-in specialists.
Going to watch is the activity, and on a big day it is genuinely extraordinary.
Winter walking, which is the real answer
Portugal's south is mild in winter and unbearable in summer, which inverts the usual season.
The Rota Vicentina — the Fishermen's Trail along the southwest coast, walking clifftop paths above empty Atlantic beaches. Far better in winter than in August.
The Via Algarviana, crossing the Algarve inland from the Spanish border to Cape St Vincent.
The Alentejo and Serra da Arrábida, both too hot from June.
Peneda-Gerês, the national park in the far north, which is green, wet and dramatic — and cold in winter, with snow on the tops.
Surfing more generally
Portugal's surf is at its most consistent in autumn and winter.
Peniche and Supertubos for the heaviest waves.
Ericeira, a World Surfing Reserve with a concentration of quality breaks in a short stretch.
The Algarve's west coast around Sagres, which works when the south is flat and which is cooler and emptier than the resort coast.
Wetsuits year round.
Climbing
Sagres and the Algarve's limestone — best in winter, when the rock is cool and the weather is settled. Deep water soloing on the cliffs in the warmer months.
Sintra's granite boulders and crags.
The Azores
Worth including because they are a year-round proposition and almost nobody goes.
Volcanic islands in the mid-Atlantic with crater lakes, hot springs, canyoning and hiking on every island.
Whale and dolphin watching is best from spring to autumn, and the Azores are one of the best places in the world for it.
Pico, the highest mountain in Portugal at 2,351 metres, is climbable with a registered ascent.
Climate is mild and wet year round, and far less visited than Madeira.
Madeira
The levada walks — irrigation channels with paths alongside, contouring through laurel forest, and one of the best walking networks in Europe.
Some involve tunnels and exposed sections with cables. Mild year round.
Practical
A car for all of the above outside the cities.
Portugal's north rains substantially more than the south — pack for it.
Winter accommodation costs a fraction of summer, and rural places may close.