The general environment is similar in hills and mountains (obviously not in valleys).

However, trees are different. i don't know much about trees, but I think they are half way to alpine woods, from the usual spanish trees.

I hope somebody can give more details on this matter, for I honestly can't tell a sycamore from a banyan. :o

As for the mountain ranges, there are some spots and short ranges where the Picos de EUropa in Cantabria and Asturias reach heights similar to pyrinees (or whatever the spelling is in English ), but if you are a trekker, Picos de Europa is a good thing but you'll regret not going to Pyrinnees.

As MM says, not only mountains are beautiful, also plains, with woods and prairies or hills and valleys...