Design an object-oriented parking lot with multiple levels, multiple spot sizes (motorcycle, compact, large), and dynamic pricing. A ticket is issued on entry and used to compute the bill on exit.

Design a library system that tracks books, members, check-outs, holds, and fines. A single title may have many physical copies; members can place holds when all copies are checked out.

Design an elevator control system for a building with N elevators and F floors. Riders press hall buttons on each floor (up/down) and car buttons inside each elevator. The system must dispatch efficiently and serve every request.

Design a vending machine that holds inventory across slots, accepts coins and bills, dispenses a product, and returns correct change. Model the full transaction lifecycle including cancellation.

Design an ATM that authenticates a cardholder, then supports balance inquiry, withdrawal, deposit, and transfer. Enforce per-account daily withdrawal limits and machine cash limits.

Design a chess game: model the board, pieces, legal-move generation, turn order, and game-end detection (checkmate, stalemate, draws). Focus on a piece hierarchy that's easy to extend.

Design an Amazon-style online bookstore: catalog, search, cart, checkout, orders, and payment. Focus on the domain model and how state moves through the order lifecycle.

Design a hotel reservation system that supports searching available rooms by date range and room type, booking with pricing, modifying or cancelling reservations, and check-in/check-out at the property.

Design an in-process pub/sub system: publishers post messages to topics, and subscribers receive messages from topics they subscribe to. Support multiple subscribers per topic, retention, and delivery guarantees.

Design a movie ticket booking system: theaters with multiple screens, movies showing at scheduled times, seat selection with temporary holds during checkout, and payment that confirms the booking.