These stories about Ravana come from the Uttara Kanda, which is the final book of Valmiki's Ramayana. It describes what happens after Rama and Sita return to Ayodhya. In addition to learning about what happens to the royal family (all the drama of Rama exiling Sita, the birth of their twins, etc.), there is also a lot of backstory. Sages come to visit Rama and they tell him stories about Ravana which add new meaning to the Rama's own dealings with Ravana, and I have selected some of those Ravana stories to include here.
Sumali's daughter Kaikasi gave birth to a child, the son of her husband Vishrava, a brahmin. So, the child was both a brahmin, but also a rakshasa, like his mother. He was dreadful to behold, having ten heads and ten necks, and he had ten pairs of arms.
Indra rained blood at his birth, and meteors fell from the sky. Jackals howled, flames exploding from their mouths, and vicious beasts of all kinds raced in ill-omened circles around the mother and child.
Vishrava gave his son the name Dashagriva, Ten-Necked, but later he would be called Ravana, the One-Who-Roars.
Jealous of his half-brother Kubera's wealth, Ravana vowed to conquer him and take his riches. He fasted and prayed for ten thousand years, chopping off one head every thousand years and casting it into the sacrificial fire.
He was about to cut off his last head when his great-grandfather appeared, the god Brahma. "Ask any boon!" said Brahma.
"Protect me from devas, danavas, yakshas, and nagas." The list went on, but Ravana did not include men, for he considered them unimportant.
"So be it!" said Brahma, and he also restored Ravana's lost heads.
As Ravana explored the mountain forests, he saw a beautiful woman practicing austerities at a fire-altar, clothed in a deer-skin, her hair in twisted jats.
"My lovely lady," he said, "you should be wearing silks and jewels, not this ascetic garb. Come live with me and be wife!"
"I am Vedavati," she replied, "and I am dedicated to Vishnu; I will marry no other."
Ravana grabbed her hair, but Vedavati magically cut herself free.
"With my dying words I curse you, Ravana," she said as she threw herself into the flames. "And I will be born again for your destruction!"
One night Ravana found Rambha wandering in the woods; she was a beautiful apsara, and Ravana burned with lust. He grabbed her and whispered, "How lucky for you we have met!"
Rambha protested. "You are my elder, my in-law; Nalakubara, your nephew, is my husband. Let me go!"
But Ravana did not listen, and he raped Rambha that night.
When Rambha told Nalakubara what Ravana had done, he cursed his uncle. "If you ever touch another woman against her will, your head will split into seven pieces."
That is why Ravana had to seduce Sita; he could not force her.
Ravana was sacrificing to Shiva on a riverbank, singing and dancing in Shiva's honor.
Meanwhile, Kartavirya, Thousand-Armed King of the Haihayas, was relaxing in the river with his wives. As Kartavirya splashed, the waters rose and the flood carried away Ravana's offerings.
Enraged, Ravana fought with Kartavirya in a ferocious battle. Finally, Kartavirya knocked Ravana to the ground, and the gods watching from heaven rejoiced.
Ravana's grandfather Pulastya pleaded with Kartavirya to let Ravana go. Out of respect for the great sage, who was a mind-born son of Brahma, Kartavirya agreed.
Ravana was strong, but there is always someone stronger.
Ravana went looking for Vali, the monkey-king, wanting to fight him. He found Vali on the ocean's shore, engaged in worship.
Ravana planned to sneak up on Vali from behind, but Vali grabbed him and shouted, "Got you!"
He then tucked Ravana under his arm and soared up into the clouds. Down below, the rakshasas saw their king struggling to get free. They chased Vali but could not catch him.
After flying around the world, Vali landed and let Ravana go.
"I want to be your friend, great monkey!" said Ravana.
Vali laughed and agreed; thus Vali became Ravana's ally.
Author's Notes.
1. The Birth of Ravana. This is from Sarga 9. The ominous signs reported here are very similar to the signs reported at the birth of the antagonist of the Mahabharata, Prince Duryodhana.
2. Ravana and Brahma. This is from Sarga 10. At first, Ravana asks for immortality, and Brahma explains to him that he cannot grant that boon, so Ravana then asks to be invincible, listing all the creatures from whom he wants protection. His failure to ask for protection from humans is what will allow Vishnu to be born as a human, Rama, who can defeat Ravana. The image I've used here is not actually of Ravana; this is a three-headed rakshasa, Trishira, another of Ravana's brothers, who makes a similar sacrifice; details at Wikipedia.
3. Ravana and Vedavati. This is from Sarga 17. As promised, Vedavati will be reborn as Sita, once again dedicated to Vishnu (Rama), and securing Ravana's destruction. The story has lots of other great details! Vedavati tells Ravana the story of how all manner of creatures asked her father to marry her, but he refused them all, vowing that she would marry Vishnu. A daitya named Shambhu was so angry at this refusal that he killed her father, and her mother threw herself onto her father's funeral pyre, leaving Vedavati an orphan.
4. Ravana Rapes Rambha. This is from Sarga 26. Nalakubara is the son of Ravana's half-brother Kubera; for more about Kubera, see Wikipedia.
5. Ravana and the King of the Haihayas. This is from Sarga 31-33. This episode covers several chapters, with the fight between Ravana and his ministers versus King Arjuna and his ministers told in elaborate detail. You can read more about Kartavirya Arjuna at Wikipedia.
6. Ravana and Vali. This is from Sarga 34. Vali's wife Tara plays a role in the full version of the story because Ravana first goes to Kishkinda; Vali is not there, but Tara speaks with him and warns him that Vali is sure to defeat him.
Bibliography. I chose these stories from the Uttara Kanda while reading the translation by Arshia Sattar, Valmiki's Uttara Kanda: The Book of Answers. In the notes, I have linked to an online translation of the Uttara Kanda by M. N. Dutt.