Including fragments is only one way a writer can create sentence structure variety. Another is to begin sentences with something other than their subjects (or the attending adjectives). Possible sentence openers include prepositional phrases (e.g. in the house), participial (words that are verb forms) phrases (e.g. waving her hand), adverbs, and even conjunctions.
Here are some examples, taken from Stephen R. Lawhead’s latest novel, Tuck:
At the cookhouse, he begged a bite to eat and a cup of something to drink, and found the kitchener most obliging.
Stepping past the gaoler [jailer], Tuck pushed the door open farther and relieving the porter of his torch, entered the cell.
A series (and mixture) of phrases;
Upon reaching the foot of the fortress mound, Tuck worked his way along the rising, switchback path towards the entrance.
Single word adverb:
Again, a slight hint of a grimace crossed the earl’s face.
That night at supper, Bran baited and set the snare to catch Wolf Hugh.
And while he told himself that paying monks to pray souls from hell was a luxury he could ill afford, dep in his heart of hearts he knew only too well …
A key point to remember is that varying your sentence structure is something to do during revision, not something to worry about in your first draft, or even in your first rewrite. First get the story down, then work to pretty it up!