Write Sentences Differently in Terms of Grammar
In English, there are twelve different verb tenses you have available, which therefore leads to twelve different types of sentences (deep breath):
Simple Present: I eat dinner at six o'clock precisely
Simple Past: I ate dinner at six o'clock precisely
Simple Future: I will eat dinner at six o'clock precisely
Present Progressive: I am eating dinner at six o'clock precisely
Past Progressive: I was eating dinner at six o'clock precisely
Future Progressive: I will be eating dinner at six o'clock precisely
Present Perfect: It is six o'clock precisely and I have eaten dinner
Past Perfect: By six o'clock precisely, I had eaten dinner
Future Perfect: By six o'clock precisely, I will have eaten dinner
Present Perfect Progressive: It is six o'clock precisely and I have been eating dinner since five
Past Perfect Progressive: It is six o'clock precisely and I had been eating dinner since five
Future Perfect Progressive: I will have been eating dinner for an hour by six o'clock
When experiencing writer's block (even when only writing a sentence), select a previously written sentence and rewrite it with a different tense. After eleven days of doing this, you will have probably thought up a different sentence to write.
Write Sentences with Different Narrators
One of the first things novel writers have to decide about their story is what point of view (POV) they want to use. The most commonly used are:
The first person: I ate dinner around six o'clock
The third person: Johnny Tremain at dinner around six o'clock
(There is a second person, but using it in a novel is awkward, distracting, and discouraged)
Just because you wrote a sentence in the third person one day doesn't mean that it has to stay that way. Try switching the POV of your sentence. You may like it better than the original.
Write Sentences that Rhyme with Previous Sentences
I ate dinner around six o'clock
Then took a stroll around the block.
They caught me and tied me to a chair,
And I got a cramp while sitting there
To me, it either sounds like the beginning of a story or third place in a fourth grade poetry competition. Either way, have fun with this one. Even if these sentences never make it into your final draft, you might enjoy the experience.
Write Differently Every Day
Nothing beats a rut better than variety. While there's nothing wrong with trying these exercises all with the same sentence one day, consider making these into a schedule such as this one:
Monday: Write sentence
Tuesday: Alter the tense
Wednesday: Alter the POV
Thursday: Write a rhyming sentence
Friday: Write a new sentence
Saturday: Alter the tense
Sunday: Write a rhyming sentence
Since altering the POV and altering the tense are relatively short activities compared to writing a rhyming sentence, consider writing the sentence in all the tenses when doing both.
And that is how to add variety to your writing workout when writing a sentence gets boring. Or, I suppose, you could just write a different sentence from a different story . . . if you wanted to.

