Pick a problem and design the architecture.
Design a service like bit.ly that shortens long URLs and redirects on lookup.
Design a rate limiting service shared across multiple API instances.
Design a system that serves a personalized home feed, like Twitter's timeline.
Design a platform like YouTube where uploads get transcoded and streamed smoothly to any device.
Design a messaging system where online users get instant delivery and offline users get notified.
Design the dispatch core that matches riders with nearby available drivers in real time.
Design a checkout flow that charges customers via an external processor without double-charging or losing consistency.