Toledo packs more monuments per square kilometre than almost anywhere in Spain. The old city is a UNESCO World Heritage Site and almost every street turns up something worth pausing for — a carved doorway, a Mudéjar tower, a Roman arch embedded in a later wall.
The main sights are clustered in two areas: the historic centre around Plaza de Zocodover and the Cathedral, and the Jewish Quarter to the south-west, where the Sinagoga del Tránsito, Santa María la Blanca, and the Monastery of San Juan de los Reyes cluster within easy walking distance.
The Toledo Cathedral and the Alcázar are the two unmissable stops. Cross the river to the Mirador del Valle for the classic panoramic view. The Iglesia de Santo Tomé holds El Greco's Burial of the Count of Orgaz — one of the great paintings in Spain, and essential if you care about painting at all.
Even if you normally have doubts about tourist trains, the Trainvision is worth considering if you have only a few hours in Toledo — it covers the panoramic route efficiently and stops at the best viewpoint.