Build a House of Cards
Take a deck of playing cards and turn it into a freestanding structure using nothing but balance, friction and a steady hand. Start with simple pairs of cards leaning against each other, then connect them with horizontal cards to build a wider base. Once that holds, add another level and see how high you can go before the whole thing collapses. The challenge is more technical than it first appears. Cards need to meet at consistent angles, the base has to be wide enough to support the weight above it, and even a small knock to the table can bring down several minutes of work. Smooth or heavily coated cards can be harder to use because they slip more easily, while a stable surface and still air make a noticeable difference. Begin with a basic two-storey structure and increase the height as your control improves. Work somewhere without fans, pets or people bumping the table, and resist the urge to rush the final few cards. Add this to your list and keep rebuilding until you can make a proper multi-level house that stands on its own.