Portugal’s network of pousadas da juventude (youth hostels) is part of the HI network. An HI card from your hostelling association at home entitles you to the standard cheap rates.
A student card will get you reduced admission to almost all sights. Likewise, those aged over 65 with proof of age will save cash.
If you plan to do a lot of sightseeing in Portugal’s main cities, the Lisboa Card and Porto Card are sensible investments. Sold at tourist offices, these cards allows discounts or free admission to many attractions and free travel on public transport.