Great project to get started with…
For no extra effort, you could make the buttons 5/10/30/60 minutes, and for each short press they’d accumulate the button value.
If you make a mistake, you make a ’long’ press on that button to subtract that value.
For giggles sake, put a simple LED 7-segment display to show the remaining time as set.
All on batteries plus a buzzer, and away you go.
If you document it well, it’s worthy of a trip to the gallery section.
Version 2.0 could use a rotary encoder with acceleration to adjust small or large values… an RTC to display when it’s not counting down, and… more.