Teaching
Twenty trading days at a news-stand. Commit to an order every morning before the day's demand arrives, and the ledger fills in beside you. At the close it values the twenty quantities you actually chose against the demand distribution, so you can see what your judgment was worth once the luck of the draw is taken out.
A service desk, one queue, and two dials for randomness. Set the desk to serve faster than customers arrive and nobody ever waits — the line stays empty at any utilisation below one. Turn the randomness up, changing neither the arrival rate nor the service rate, and the queue appears. Runs on a simulated clock you can wind from a minute a second to a day a second.