Spain gives away a lot, and two specific national customs — free museum windows and, in a few cities, free food with drinks — make a cheap trip genuinely cheap rather than merely frugal.
The free tapa belt
Granada is the best known: order a drink and a substantial tapa arrives with it, unasked and unbilled. Four or five drinks over an evening is dinner.
Jaén and Almería follow the same custom, and parts of Andalusia do it inconsistently.
Elsewhere — Madrid, Seville, Barcelona — tapas are ordered and paid for like any dish.
This single custom makes Granada one of the cheapest cities in western Europe for an evening out.
Free museum hours
The Prado is free in the final hours of the day, on a published schedule. The Reina Sofía operates a similar window.
Many regional and municipal museums do the same, on specific afternoons or evenings.
Expect queues — the practice is well known locally. And check current times, because schedules are adjusted and old information circulates widely.
Done consistently across a trip this saves €15–20 a day.
What must be paid for
The Alhambra. Timed entry to the Nasrid Palaces with capped numbers, and it sells out weeks ahead. There is no workaround and no cheap alternative.
Book direct on the official site. Resellers charge substantially more for the same slot, and some sell tour packages where the entry is the only real component.
Sagrada Família and Park Güell, both timed entry. Tower access at the Sagrada Família is separate.
The Mezquita in Córdoba, which has a free early-morning window on some days — worth checking, and it is not well publicised.
Free and excellent
The Albaicín in Granada, and the Mirador de San Nicolás for the Alhambra view at sunset.
Seville's Plaza de España, Parque de María Luisa, and the Metropol Parasol from below.
Barcelona's Gothic Quarter, the Born, Gaudí exteriors, the beach, and Montjuïc.
Madrid's Retiro park, the Crystal Palace inside it, and the Templo de Debod at sunset — an actual Egyptian temple given to Spain and reassembled in the city.
Every historic centre in the country. Toledo, Salamanca, Cáceres, Santiago, Girona.
The menú del día
Three courses with bread and a drink for €12–16, weekday lunchtimes, in ordinary restaurants everywhere.
The same food à la carte at dinner costs two or three times as much, because Spaniards eat their main meal at lunch.
A place without a menú board is aimed at visitors.
Where the trip gets cheap
Andalusia's interior, Extremadura, Castile and León, inland Galicia.
A good restaurant meal in Cáceres or Salamanca costs what a plate of tapas costs in central Barcelona, and accommodation runs 30–40% below the coast.