The Murders at Karlov Manor Deep Clue Sea Commander Deck includes 1 Green-White-Blue deck of 100 Magic cards (2 Traditional Foil Legendary cards, 98 nonfoil cards), a 2-card Collector Booster Sample Pack (contains 1 Traditional Foil or Nonfoil alt-border card of rarity Rare or higher and 1 Traditional Foil Borderless Common or Uncommon card), 1 foil-etched Display Commander (a thick cardstock copy of the commander card with foil etched into the cards border and art), 10 double-sided tokens, 1 deck box (can hold 100 sleeved cards), 1 Life Wheel, 1 strategy insert, and 1 reference card.
If they play Commander, a commander precon deck is another strong choice
Or if they were suspected you can exile and end their existence instead
Elesh Norn, Mother of Machines All hail Phyrexia
Regardless of your cube design I think you can safely take it to 20% land count
Suppose you want to make a play before your next turn and have two draws and a scry