This takes place sometime during the episode "Fairy Fairy Quite Contrary" when Cosmo learns Juandissimo (Wanda's ex boyfriend) is in town. Cosmo is clearly jealous in that episode and I think he would be more romantic as a means to keep Wanda in love with him.
XxX
Timmy's bedroom door slammed open as Timmy, Cosmo and wanda came in. Wanda looked exasperated while Cosmo floated in with a smile and Timmy happily reminisced about the last hour..
"Oh man," he said with a chuckle. "I can't believe the firecracker shot that far!" He exclaimed as he flopped down on his bed.
"Yeah, well maybe next time you should wish for something a little less dangerous." Wanda said floating next to Timmy with her eyes narrowed at him.
"Awe, come on Wanda. It wasn't that bad!" Timmy chuckled at how frazzled she was.
"Yeah, it could have been worse. At least it didn't land in the woods like last time." Cosmo piped in.
Wanda rolled her eyes and sighed. Boys!
"Well, I suppose it's time for you to study for your biology test." She said as she poof'd up Timmy's biology book and notes.
"Awe! Do I have to? Cosmo, let's play the 'not study game'"
Wanda gave Cosmo a weary look as she rubbed her temples. It had been a long day and Timmy really needed to study.
"Sorry Timmy. I can't play tonight. I have something to do." Cosmo said to his godchild.
Timmy and Wanda looked at him curiously. Wanda was about to question him but decided against it. If Cosmo was occupied, her chances of getting Timmy to study were a lot higher.
"What do you have to do, Cosmo?" Timmy asked.
"Oh, um, it's..." he gripped his wand nervously as he tried to think of how to not ruin the surprise. "What's that, Phillip? You need me?" Cosmo quickly kissed Wanda on the cheek.
"See ya, Timmy." He poof'd into the castle quickly.
Timmy and Wanda looked at the fishbowl curiously. Then Wanda remembered Timmy needed to study.
"All right sport, lets start studying!"
Timmy groaned as he made his way to his desk.
xxxx
Inside the castle, Cosmo was in his bedroom working on a special surprise for Wanda. He was bustling around the room lighting candles and scattering rose petals all over the place. He dimmed the lights and put on some soft romantic music in the background.
When he was done, he made his way into their bathroom, where he ran a nice hot bath. He dropped in one of Wanda's favorite scented, bubbling bath bombs and watched in amusement as bubbles started forming immediately.
He placed a little tray of Wanda's favorite chocolates on one corner of the bathtub and her favorite scented candle in another corner.
Once that was done, he poof'd into their kitchen where he quickly poof'd up some of Wanda's favorite foods and lit more candles on the table.
As soon as he was done, Wanda came inside the castle.
"Cosmo?" She called from the living room.
Cosmo quickly went into the living room with a bouquet of roses in hand.
"Hey Wanda," He kissed her on the cheek then handed her the roses.
"Oh, Cosmo!" She gasped then admired her bouquet. "What's the occasion?" She asked playfully.
"I just love you." He smiled and shrugged. "Are you hungry?"
Wanda eyed him suspiciously.
"A little..."
Cosmo took her by the hand and lead her into the kitchen. Wanda was surprised to see her all of her favorite foods.
"Cosmo, this is really wonderful."
Her husband smiled at her before leading her to a chair. He pulled it out for her and pushed it in.
"Nothing but the best for my wife." He winked at her, sitting in the chair next to hers.
They spent the next several minutes eating and chatting about nothing in particular. Right as they got done eating, Cosmo remembered he had a special gift for Wanda.
"I have something for you, close your eyes."
Wanda eyed him skeptically. She wasn't used to her husband being so secretive, or this romantic. But tonight he was surprising her. But what was the occasion? It wasn't her birthday, or their anniversary.
"Please, Wanda?"
"Okay..." she closed her eyes to humor him.
Cosmo retrieved a small velvet box, containing the ring he got for Wanda earlier that morning. He opened it and set it on the table facing his wife.
"Okay, open your eyes." He said before holding his breath.
Wanda opened her eyes and looked at him curiously before following Cosmos gaze to the table. When she saw the ring, she gasped and brought a hand to her mouth.
The ring was beautiful. A heart shaped diamond was set in the middle of a silver band, and the word 'forever' engraved around it.
"Cosmo... what is this?"
Cosmo took the ring out of the box and slipped it onto her ring finger, in front of her wedding ring.
"Do you like it?" He asked hopefully
"Cosmo, it's beautiful. But, what's the occasion?" She asked admiring her ring.
"I never got you an engagement ring and even though we're already married, I just thought you should have one." He smiled sweetly
"Cosmo, I love it! But you really didn't have to get me a ring."
Cosmo looked at her and smiled widely.
"Well, I really think you should have it. You deserve it."
"You're so sweet!" She said gently as she leaned in to kiss him.
"Wanda," He said breaking the kiss. "I have another surprise for you."
He poof'd them into their bathroom where the hot bath was waiting.
"Cosmo, what is all this?"
"I just want you to relax. I lit your favorite candle and your favorite chocolates are on the plate in the other corner." He pointed to the chocolate.
"Cosmo, I... thank you so much for all of this. But you really didn't have to go through all this trouble for me." She wrapped her arms around his waist and looked into his eyes.
"I know, but I wanted to, Wanda. You just mean so much to me."
She leaned into him and kissed him passionately before resting her forehead against his.
"Are you going to join me?" She asked with a sly look.
"D-do you want me to?"
Wanda kissed him again, hoping that would answer his question.
"Cosmo? Wanda?" The two fairies pulled apart and Wanda sighed.
"I guess we better go see what Timmy needs."
"I'll go! You stay here and relax." Cosmo insisted.
"Are you sure?" Wanda asked
"Yes." He kissed her again "I'll be back soon." He brushed his fingers over her cheek before proofing off.
Wanda smiled and turned to look at the bath before undressing to get in. Once she was inside the tub, she leaned her head back and relaxed. Then she realized something. They recently found out Juandissimo was in town, godparenting. Was Cosmo doing all of this because he was jealous? A large smile spread across her face.
"I'll just have to reassure him when he comes back." She said to herself.
Then she indulged in the plate of chocolates as she waited for her husband to return.
