Excellent choice! Salamanca is a lovely medieval university city, and Toledo is undeniably one of Spain's most historic city.
Toledo is closer and one can make it a one day tour, however, the scenery from the historic parador is worth an overnight stay.
Salamanca is also worthy of an overnight stay. There's nothing lovelier than seeing the sun set bathing the oncient stones of the Catedral. The Plaza Mayor is monumental, and while not as large, in my opinion as opulent as the Plaza Mayor in Madrid. You may be able to take in a quick look at Avila since it is on the way to Salamanca.
Buen viaje!
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The world is a book, and those who do not travel read only a page.
--St. Augustine (354-430)