Lily and Max found themselves standing wide-eyed in Trafalgar Square, surrounded by the cacophony of honking cars and chatter. Their parents were nowhere in sight, having inadvertently left them behind during their vacation exploration. "Max, what are we going to do?" Lily's voice trembled slightly as she clutched her brother's arm. "Don't worry, Lily! We'll figure it out. Let's start by heading towards the river. I remember the hotel was somewhere near the Thames." Max tried to sound confident, though his heart raced with uncertainty.

Max and Lily paused to watch a juggler, his colorful pins spinning through the air in a mesmerizing dance. As they moved on, the aroma of roasted chestnuts and fish and chips beckoned them from nearby stalls. "I wish we could stop and eat, but we should keep going," Lily sighed, glancing longingly at the steaming food. "Maybe later. Let’s make sure we find our way back first," Max replied, scanning the crowd for any sign of familiarity.

Oliver, a mysterious street artist with a penchant for riddles, noticed the siblings and called them over with a wave. "You two look a bit lost," Oliver remarked with a knowing smile. "Perhaps I can help, for a price—a riddle to solve!" "A riddle?" Lily asked, curious despite her urgency to get home. "Indeed! Solve it, and you might just find your path," Oliver said, his paintbrush still dancing over the canvas.

"In a city of fog, where the ravens guard, find the crown where Time keeps guard. What am I?" Oliver whispered, his eyes glinting with challenge. Max furrowed his brow, thinking hard. Lily bit her lip, her mind racing through the landmarks they had visited. "The Tower of London!" Lily exclaimed suddenly, her eyes lighting up. Max nodded, recognizing the answer.

As they walked, the siblings held hands, their pace quickening. The towering fortress finally came into view, its ancient stones bathed in the glow of nearby lanterns. "We did it, Lily! We found it!" Max cheered, relief washing over him. "I knew we could," Lily replied, her smile radiant against the night.

Lily and Max ran forward, wrapping their parents in tight hugs. The night sky above them sparkled with stars, a fitting end to their unplanned adventure. "Maybe next time, we can explore with a map," Max joked, earning a laugh from everyone. "Or with Oliver leading the way," Lily added, glancing back at the city with a secret smile.