If you can afford it, i would spend 1/2 nights in Sevilla. It is among the beautiest cities in Spain, and is completely different from Madrid and Barcelona. Though expensive, it's only 2 and a half hours from Madrid by train. 8 days is quite enough for Barcelona-Madrid-Sevilla, and you will see three different types of Spanish cities.
If not, 1 day trips recommended above are good alternatives too (Segovia better than others)
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