Personally, this is one of my favorites :) Enjoy!

Headcanon 4: Once when Lin and Tenzin left the Airbending household, Tenzin was asked if he had kissed Lin by Kya since when playing hide-and-go-seek, they had been gone a long time. Tenzin and Lin denied it. Toph sensed Tenzin and Lin were lying, and immediately told Aang.

"I'm bored." Lin stated.
Bumi, Honora, Kya, Lin, and Tenzin were laying in the grass on Air Temple Island. It was a beautiful day out and none of them really knew what they should do.
"We could throw stuff at the Air Acolytes again," Bumi suggested.
"Nah, we did that yesterday." Lin replied.
"Sparring?" Honora said.
"No, we sparred this morning," Kya groaned. "I'm still in pain from Lin's earthbending."
"Sorry..." Lin grumbled.
"I know!" Bumi declared, sitting up. "Let's play hide-and-go-seek!"
The gang groaned.
'Bumi, we're teenagers now," Tenzin said. "We're too old for that game."
"Come one guys! It'll be fun!"
"Ugh, I'm so bored, I guess I'll play..." Lin agreed.
"Fine, let's play." Tenzin quickly sided with Lin.
Kya and Honora finally agreed and the 5 teenagers stood up.
"Kya's it!" Bumi exclaimed.
"What? Oh fine..."
"ONE! TWO! THREE!" Kya shouted.
Quickly, Honora, Bumi, Lin, and Tenzin darted away from her in different directions.
"Lin, follow me!" Tenzin hissed towards her, outstretching his hand, beckoning her to take it. "I know the perfect spot!"
Lin took his hand and together, the two 15 year olds ran towards the trees.
Tenzin stopped at a very large tree and quickly airbent the base of the tree. A hole appeared and Tenzin turned to smile at her.
"Remember when we used to hid here when we were kids?"
"Of course I do!" Lin chuckled. "I'll go in first."
Lin slipped into the hole and found herself surrounded by earth and darkness. It was a tight squeeze for her.
"Tenzin, I think..."
She was too late. Tenzin had already slipped into the hole with her and sealed off the opening, cutting out the only light. Lin felt Tenzin's body press up against hers from being so cramped.
"Whoa, it's a lot smaller than I remember."
"Or maybe we outgrew this spot." Lin said. "We should find a new hiding spot."
"Agreed..."
Tenzin was about to bend the opening back off when they both heard Kya shout, "READY OR NOT, HERE I COME!"
"Crap." Lin mumbled. "I guess we're stuck here."
There was an awkward silence between the two. Lin could feel Tenzin's heart thudding, not only with her seismic vision, but because she was so close to him. it was almost impossible to move because the space was so small.
"Why did I come with you?" Lin mumbled.
She heard Tenzin chuckle in the darkness before silence took over again.
After a few minutes, Tenzin cleared his throat.
"Lin, there's something I've been meaning to tell you..."
"Is now really the best time to tell me this?"
"Um, well... no.."
"Go ahead."
"Well, um, Lin... I... We... Er..."
"Spit it out, Airhead."
"Lin, I... well..."
Lin felt his lips press gently against her own. His gesture took her by surprise and she felt her eyes flutter shut. His lips were soft and gentle, but they only lingered for a few seconds before he pulled away.
"Well, that."
Lin smiled in the darkness, knowing that Tenzin couldn't see her. She had been waiting for him to kiss her for so long, though she didn't expect it to be under a tree in a hole.
"I'm sorry, I didn't mean to..." Tenzin stuttered nervously.
"Tenzin. Shut up."
Lin wrapped her arms around Tenzin's neck and guided him back towards her lips. She kissed him with all her heart, trying to put all of her emotions and feelings into it. Tenzin kissed her back. She felt his hand clutch her cheek before it moved to gently run through her soft, ebony hair. the kiss lingered for a minute before Lin gently pulled away.
There was a moment of silence before the two burst out laughing. Their lips met again and everything faded away. They forgot they were playing a game. They forgot that they were in a cramped hole beneath a tree stump. All that mattered was them.
A while later, Tenzin said,
"We should probably head back. They'll be wondering where we went."
"Okay," Lin said, almost sadly.
She didn't want this moment to ever end.
Tenzin quickly bent the opening back, allowing light to enter. He pulled himself out and turned around, holding out his hand towards Lin. She took it and he pulled her out of the hole and to her feet.
"Thanks." She smiled, placing a small kiss on his cheek.
Tenzin returned the smile, before his expression turned to panic.
"Lin, look! The sun is starting to set. We were down there for hours."
"Oh crap, we are so dead."
Together, they ran towards the house and burst through the front door. Waiting for them in the kitchen were Kya, Bumi, Honora, Katara, Aang, and Toph.
"It's about time you two showed up!" Katara said. "Where were you?"
"We were playing hide-and-go-seek and Kya never found us." Lin explained.
"Well, I guess that's fine." Katara stated. "All of you, go wash up for dinner."
The teenagers all went towards the bathroom to wash up. When they were walking back, Kya asked Tenzin privately,
"You kissed her, didn't you?"
"What? Where'd you get that idea?" Tenzin demanded, his voice hushed.
"Tenzin, I'm 18. I know these things." Kya chuckled. "Did you?"
"Of course not! Lin and I are just friends."
"Mhmmm." Kya smirked before going to join the girls, leaving Tenzin behind with his face bright red.
Dinner went smoothly. Kya kept smiling at Tenzin and Lin. But Kya wasn't the only one that knew of Lin and Tenzin's actions. Toph could feel their heart beats skyrocketing and she knew immediately that something had happened between the two.
After dinner, Lin and Toph bid their goodbyes and headed towards the ferry. Once the ferry had pulled away from the dock, Toph turned to her daughter.
"So you and Twinkletoes Jr. kissed, didn't you?"
"What? Mother! Tenzin and I are just friends!"
From the tone in her voice, Toph knew she was lying, but she decided to mess with her.
"Okay, whatever you say, darling."
The rest of the ferry ride was silent, and when they got home, and Lin had gone into her room for the night, Toph called up Aang.
"Avatar Aang speaking."
"Prepare the invitations, Airhead. The love birds finally kissed,"