Numbers Up! is ostensibly a dexterity stacking game from Atomo where each of the 2-4 players has a set of large chunky brightly coloured plastic numbers 1-5 to stack. There's more to it tho' because designer Juan Carlos Ruiz has incorporated a variety of challenges in the cards that players have to satisfy.
Players are racing each other to score points (purple plastic cubes) for being the first to complete the requirements of the cards that are flipped. Some cards are super easy; for example, showing the five colours or five numbers and so satisfied merely by stacking the numbers in a column in the order shown on the card. Others tho' demand a tad more thought because the column has to be built with a specific number omitted or obscured behind another number. And then there are the cards that incorporate a mathematical challenge: an equation or just displaying a number. These have to be satisfied by stacking the numbers used to represent and solve the equation or those needed to arrive at the target number using any of the four basic arithmetic operations (addition, subtraction, multiplication, division) in any combination.
None of the cards are individually very difficult but you'll need to combine a steady hand with a degree of mental agility to switch instantly between cards demanding stacking by colour, those requiring a specific number to be obscured or excluded, and those calling for a mathematical calculation to determine the numbers to be stacked.
If you want to further add to the challenge of Numbers Up!, there's scope for incorporating your own house rules. You could consider handicapping players by requiring them to use only one hand for stacking. In our plays at Board's Eye View we also added to the challenge of the maths cards by flipping the numbers to indicate the arithmetic operations: straight up for addition, upside down for subtraction, rotated 90º to the right for multiplication and 90º on the left for division...