Yosemite is a national park with truly amazing scenery and half-tame bears.
Yosemite is a national park with truly amazing scenery and with half-tame bears.

Stylistically, it is not necessary to repeat the preposition, with, because it sits very close to the first one.

Every weekend I go to the gym, play basketball with my friends and go for a hike.

Each phrase needs to contain its own verb to complete the parallel structure of the sentence; otherwise the sentence is grammatically incorrect.

The comedian was hilarious, but very rude.

The original sentence is not grammatically incorrect and would be acceptable in spoken English, but the style is awkward on paper, because the sentence is not structured in a parallel way.

The best and most beautiful things in the world cannot be seen or even touched. They must be felt with the heart.  (Helen Keller)

A parallel structure is achieved by making the second verb passive like the first one.

Gaugin was a remarkable painter, sculptor and writer.

The independent clause - he was also well-known for his writing - has been replaced with a noun to accompany the first two nouns.

Manatees are very large animals, completely aquatic and vegetarian.

Notice the use of three adjectives to make the sentence parallel.

The boys’ football shot over the fence, bounced down the road and fell into/landed in a bed of roses.
The boys’ football shot over the fence, down the road and into a bed of roses.

The first answer is parallel because each phrase begins with a verb in the past tense.
The second sentence is parallel because each phrase begins with a preposition. Notice how the verb – shot – does not need to be repeated. 

The policeman said to the suspect, “Tell me where you were, what you were doing and who you were with at 7 o’clock yesterday evening”.

He went to live in Scotland because he had grown tired of the Far East and wanted to be near his ageing mother.

Both verbs are now in a past form.

Jones argues in his article about Disneyland that the spontaneous element of play has been removed, that the park reinforces ideas of male dominance and racism, and that the theme park is out of place in countries like China and Japan. 

The repetition of that makes it much clearer to the reader that the article contains three different arguments. These, and the commas, also help to define where each one begins and ends.

We must either increase our revenues or reduce our costs.
Our revenues will either need to be increased or our costs reduced.

Note the use of ellipsis (or the removal of unnecessary words) from the second answer. Without the use of ellipsis the sentence would be unnecessarily repetitive and wordy, i.e.

Our revenues will either need to be increased or our costs will need to be reduced.

The football team is responsible not only to the manager, but also to the fans.
The football team is not only responsible to the manager, but also to the fans.

Her new novel is both engaging and very moving.
She has written both an engaging and very moving new novel.

I had no idea what she saw in her new boyfriend; he was neither good-looking nor was he very interesting to talk to. 

People often avoid making eye-contact with each other whether they are in a lift, on a train or in a supermarket queue.
People often avoid making eye-contact with each other whether they are standing in a lift, sitting opposite each other on a train or queuing in a supermarket.

The first answer consists of parallel prepositional phrases, whereas the second contains parallel verb phrases.

It is better to judge people by their actions rather than by their words.
It is better to judge people by what they do rather than by what they say.

The schools in the rural area are not as big as the schools in the inner city.
The schools in the rural area are not as big as those in the inner city.
The schools in the rural area are not as big as the ones in the inner city.

The schools in the rural area are not as big as the inner city – This sentence is not logical, because it compares the size of the schools with the size of the inner city. The sentence should compare schools with schools and there are various ways of doing this.

Sarah prefers to go out with just one or two friends than to socialise in a big group.
Sarah prefers going out with just one or two friends than socialising in a big group.

See how the verb forms are different in each sentence, but within each sentence they are the same and therefore parallel.

The rules of cricket are much more difficult to learn than the rules of baseball.
The rules of cricket are much more difficult to learn than those of baseball.    
Cricket is much more difficult to learn than baseball.

Christine likes yoga a lot less than Tina does.

Christine likes yoga a lot less than Tina – This original sentence compares yoga with Tina, which is probably not the intention of the writer. If we avoid this confusion by writing - Christine likes yoga a lot less than Tina likes yoga – the sentence is unnecessarily repetitive. The auxiliary, does, parallels likes, because its form is present and third person singular. Compare the use of the auxiliary, did, in sentence 3. 

In summary, parallelism is an important characteristic of academic writing, creative writing and report writing and refers to the balanced construction of sentences. In a good piece of writing this balanced structure should be achieved with words, phrases and at the level of the clauses. It is important to note that both word forms and verb tenses should be consistent when they are paired or when they are part of a list. Remember also not to mix active and passive verbs. Parallelism, however, is not only a feature of grammar, but also it affects of style. It makes your writing smoother, more elegant and with a sense of maturity more mature. Whenever you use coordinating or correlative conjunctions in your writing, or whenever you compare or you want to list items, you should be on the alert for parallelism. An understanding of parallel structure helps you write with better accuracy more accurately, more clearly, and more efficiently.