Their last day in Rome is spent shopping and exploring the vast buildings of the Vatican museum; the wings devoted to Greek history are particularly enjoyable as Percy and Annabeth recognize each statue at a glance while the other tourists debate whether that's Poseidon or Zeus, Athena or Artemis. Before it gets dark, they move on to the Pantheon, where many more statues of Romanized Greek deities reside. After they thoroughly tour every room, Annabeth stops outside to check out the massive columns at the entrance.
"Percy, Percy, Percy! Take a picture of me with the doors and the columns!"
"Oh, so now you want pictures?"
"Seaweed Brain, these are the original bronze doors! Built under Emperor Hadrian from 119 to 128 AD! They're nearly 2,000 years old!"
"Fascinating!"
"Don't make me come over there."
"Why not? I've been waiting all day for a kiss."
"Because I wouldn't be kissing you if I had to move from these doors before I got my picture."
"Is that a threat or a promise?"
"Percy, just take the picture! You've gotten to choose the pictures the whole trip."
"Please?"
"Please."
"Yes, ma'am, Wise Girl."
She poses beautifully in front of the original bronze doors for a few shots, then moves to the columns, where she tries to wrap her arms around them, but would need about three more Annabeths to make it work. The idea of any more Annabeths in the world makes Percy's head hurt; he's pretty sure it would be some kind of paradox and the planet would explode. Annabeth calls him over and he wrangles an English couple into manning the camera for him. He and Annabeth stand together in front of the column, reach their arms around but can't quite touch, and lean around the front, Percy attempting to get that kiss and Annabeth sticking her tongue out at him.
The Trevi Fountain is their last stop of the trip. It is already dark as Percy and Annabeth step out of a gelato place with cones of colorful Italian ice cream and make their way through the crowded streets to the small piazza that is home to the fountain. Pushing through to the base of the fountain, the young couple looks up and marvels at the well-lit statues, the cascading waterfalls, and the shimmering pool filled with coins from all over the world. As Annabeth points out Neptune, right in the middle of the group of statues, Percy pats his jeans pocket to make sure it is still there; the uncomfortable lump reassures him of its presence.
Annabeth pulls two pennies from her little shoulder bag and offers one to Percy. The myth is that if you toss a coin into the Trevi Fountain, you will return to Rome. Annabeth doesn't know if it's true, but if it is, she's not missing a chance to make sure she comes back. With their backs to the fountain, she and Percy stand with a penny in one hand and another hand in the other. On the count of three, they flip the coins over their shoulders and into the water. After that, it is just more dazed staring and wide-eyed admiring.
Annabeth is rambling about the history of the Trevi Fountain when she turns to Percy and her words stumble to a halt. He has one knee on the wet stones of the ground.
"Percy, what are you doing?"
"Something I've wanted to do since the day you found me at Camp Jupiter."
"And what, exactly, might that be?"
"Ask you to marry me."
"…what?"
"Annabeth, seeing you again after those seven months made me realize just how much I never want to be apart from you. And everything that happened in Tartarus… gods, you have no idea how hard it's been forcing myself to wait until now to ask you. I've been constantly terrified of losing you at any moment over these last four years. I want to know that I'll never be able to lose you to anything or anyone. But since that's not possible, this is the next best thing."
"So… you're proposing just so you can keep tabs on me?"
"No. Annabeth, I love you. I've loved you for most of my life, and I want it to be for the rest of my life. I'm all yours now, but I want to be yours forever. And I want you to be mine."
"Oh, Percy… I am yours. I always have been."
"Then will you marry me, Annabeth?"
"Yes, yes, yes!"
Grinning in that maddeningly adorable way, Percy pulls a tiny box out of his pocket, removes the diamond-and-turquoise ring, and slides it onto Annabeth's third finger. She smiles with tears in her eyes, pulls Percy up from the wet ground, hugs him tightly, and then kisses him.
"I love you too, Percy. I love you so much."
"And I love you, Annabeth."
Their last night in Rome is spent in complete blissful oblivion. As they walk back to their rooms, Annabeth keeps raising their entwined hands just to see her ring sparkling between Percy's fingers. Sleep doesn't claim either of them until they have utterly exhausted themselves talking about wedding plans. On the plane home the next day, Annabeth tucks her head against Percy's shoulder and holds his hands. His fingers play with hers, frequently going to the ring. Annabeth whispers so only he can hear her.
"I always knew I'd end up as Annabeth Jackson."
"You did?"
"Well, not always. Obviously, I thought I hated you when we first met."
"Oh, I remember."
"But ever since our first kiss, I think I've known. It just became a stronger, more sure feeling each time something happened to us. Our one-month anniversary in Paris, you disappearing, finding you again, our first trip to Rome-"
"Don't think about that, sweetheart."
"-Tartarus, the Doors of Death. All of that horrible stuff got us to right here, right now."
"I don't know about you, but I could have done without all the near-death experiences."
"Yeah. I'm just glad we've escaped enough of those to get here."
"Me, too."
"You do realize we're going to Greece for our honeymoon, right? I actually want to see it, not just dash through after closing big ominous Doors that lead straight into Tartarus, you know?"
Percy just laughs, and Annabeth joins him.