The answer depends on your understanding of 'simple'.
This calls for a small microcontroller that scans the keypad, stores the entries and controls the display.
This is a simple hardware setup which on the other hand will require some programming.
Another method would use a counter to address the digits of the display, a register for each digit of the display to store the number, a decoder to generate the 7-segment code from the binary register output, a few gates to de-bounce the keys, a clock oscillator to ontrol the timing and possibly a few flip-flops and gates for controlling the proper sequencing.
No programing required.
I leave it to you to judge what is 'simple' in your opinion.