Hello!
Where are you flying into/ out of?
You should end your trip in the city you leave from.
So, for example, if the airport is Madrid, I would head straight to Valencia by train, then Barcelona, then Madrid.
And if it is Barcelona, head straight to Madrid by train, then Valencia, then Barcelona.
The logic behind my suggested order of visit is to have the shortest train ride upon arrival.
And how many nights do you have on the ground in Spain? I'll assume 12 nights.
With 12 nights and with your interests, I would allow 4-5 nights in Madrid, 2-3 in Valencia and 4-5 in Barcelona.
If you are into museums, Madrid has a lot to offer (Prado, Reina Sofia, Thyssen Bornemisza are "must-dos" in this category), and Barcelona comparatively less, but Barcelona compensates with more visitable monuments, especially if you are interested in Art Nouveau / Modernisme architecture.
Speaking of which, I would only consider Reus if you are Gaudí "buffs". Since you mention hiking/nature, I would perhaps consider Montserrat as an alternative day trip from Barcelona.
As for Toledo, I think it is worth the day trip, but it does suffer from crowding at some times of the year. An alternative could be Cuenca: it could be nice to spend the night there after landing in Madrid, then continue on to Valencia the next day. This is more an idea for you to explore than a recommendation.