Alternative structures for "not only ... but also ..."?

Doing this isn't exclusively fun, it's one of the most important activities of human beings.

Doing this is not purely fun, on the contrary, it's one of the most important activities of human beings.

Doing this is not wholly classified as fun, conversely it's one of the most important activities of human beings.

He is not a mere teacher, he is one of the greatest educators in history.

He is a teacher and more, one of the greatest educators in history.

This is an example of a correlative conjunction.

A paired conjunction (such as not only . . . but also) that links balanced words, phrases, and clauses.

The elements connected by correlative conjunctions are usually parallel--that is, similar in length and grammatical form.

You can find a link to a list of other kinds of correlative conjunctions above.

And you can find how to deal with the problems these create here: http://grammar.ccc.commnet.edu/grammar/parallelism.htm#correlative

Doing this is fun, but also one of the most important activities of human beings.
Doing this is fun, and one of the most important activities of human beings.
He is not just a teacher; he is one of the greatest educators in history.
He is not a mere teacher: he is one of the greatest educators in history.

Between the first two sentences I wrote there is a little difference in meaning, though.