On the bench · not built yet

Jellybean Jar

Draw random samples from a giant jar to estimate the color mix; bigger samples get closer to the truth.

Nearby on the shelf

Run a diner register: tips, tax and discounts. Spot the constant of proportionality in every receipt.

Add hot air (positives) and sandbags (negatives) to a balloon; removing a sandbag goes up. Signed rules...

Memory-style card game: match expressions that are equivalent but written differently.