not sure how much latitude you have, but Crossroads offered buttons as inputs and led's as outputs.
A) you can use a 5x5 button array,
B) or 1 button that you press 25 times.
C) you could use 2 buttons, over and down and make the person press them to light the desired square.
D) 6 butttons, either one per pin (easier) or use a resistor to make a voltage divider for AI (not much harder)
using the 2 buttons or 6 buttons would need you to program them to step down through each row.
for 24 buttons you would need either a shift register to input the button data or use a voltage dividing network and connect to the analog pins.
24 LED's
3 shift registers
caps
the 5x5 LED's light up by the 595 shift register.
you can find the sketches to do most of this in the menus of the arduino software.