Princess Peach stealthily slipped through the darkened rooms of Bowser's Castle, parasol at the ready in case Ninjis were preparing to jump at her out of the shadows. She'd snuck in through the back door of the place, tiptoeing to the accompaniment of a single spotlight. Jumping over lava pits surmounted by columns, she stomped on an opportunistic Mecha-Koopa, picking it up. Holding it in front of her like a shield, she released it into the darkness. Three 'klup' sounds told her she'd knocked out three Ninjis laying in wait. There was only a little farther to go before the final door.
As she crossed the bridge and jumped through the marker tape, Peach took an internal inventory. Parasol? Check. 1-Up mushrooms? Check. Extra Fire Flower? Check.
She opened the door, and began climbing up the giant staircase that cut through the heart of Bowser's Castle, going from the dungeon entrance (where she was now) to the very, very top.
Upon the roof, Mario and Bowser had watched the sun set over the Mushroom Kingdom. A thousand bright stars came out, twinkling on a land (mostly) peaceful. Bowser had had his arm around Mario's shoulder for some time now; Mario, his arm around the lower part of Bowser's shell. As, far in the distance, a faint fireworks display from the Beanbean Kingdom caught the eye of the pair, Bowser found himself turning towards Mario. Mario, towards Bowser.
Slowly, the pair approached one another, gazing into each other's eyes.
Then, Mario and Bowser kissed, locked in a friendly hug that had turned into an embrace. Mario's lips met Bowser's for what seemed one perfect, eternal second. Then, an icy "Ah-em" brought the couple looking around, startled.
Princess Peach stood there, parasol at an aggrieved angle, hands planted squarely on her hips. Bowser took one look at her, and blushed scarlet.
"Er... ahem, er, my sweet, I'm sorry," he said, his growly voice sounding unusually embarrassed. "I would have the Clown Car all ready, but, um..."
Princess Peach smiled. "It's ok," she said, in her ever-sweet voice. She moved over to Bowser, giving him a quick peck on the cheek. "Let's do the final battle quickly, and then we can call it a night..."
What happened thereafter is something of a mystery. It's certain that Peach and Bowser faced off in the most epic showdown ever, with dramatic thunder and vertiginous lightning, with Bowser throwing wind-up toys and fireballs, and with Peach reflecting the fireballs, dodging the Mecha-Koopas, and proceeding to kick Bowser squarely into next week. It's highly likely that, once the clamor had died down and Bowser had been fished out of the pond, all three proceeded into the castle's living quarters and shut the doors behind them.
It's possible that, later that night, Peach baked a cake, and she and Bowser ate it all up. It might be true that Mario, Peach, and Bowser proceeded to swap stories, or play party games, or other things. Peach may have opened the secret compartment in the back of her closet, and brought out the dress she got when she had been possessed by the Shadow Queen. Mario may have had to deal with two 'villains' at once; or, perhaps, Bowser tried his hand at being a hero.
One could even speculate that at the end of the night, Peach, Bowser, and Mario fell asleep together in a cuddly hug, Bowser blowing tiny jets of flame as he snored.
The world will never know.
All we can be sure of is
that Princess Peach had had a great time being the Hero.
A special thanks to everyone who's reading this. Also, thanks to Mario Party and the Super Mario Wiki. I'm going to have to write my other fic sometime, with Peach as the head of the Mushroom Mafia...