What solution would be a low power option to signal a computer with button presses?
It would be nice to consider the possibility of this being a wireless solution. Either because the device is low powered enough to operate for weeks or months on a charge


To do this, you ideally want a short range radio scheme where the transmitter can be entirely powered off until a button is pushed, and then wake up just long enough to get a message through to a PC.

The chipsets used in wireless mice and keyboards are suitable for this - things like the nRF24 series.  Because the receiver is plugged into the PC, it can always be scanning, and the transmitter can wake up, send a message, wait for confirmation, and go back to sleep.  Clones/workalikes of these chips are available for around a dollar each in small quantity on modules can be wired into simple MCU boards (Arduino, etc) but you need one at each end.  An alternative is that Nordic's BLE MCU's like the nRF51822, nRF52832, nRF52840 combine the radio and MCU and can also speak simple custom protocols - the nRF52840 has the particular advantage of being directly a USB device that could plug into the PC, and if desired emulate a standard keyboard.  (There were originally versions of the nRF24 with a built in USB MCU specifically for the wireless mouse receiver role, but they may be more expensive to source and develop for today than a BLE MCU).

A second option would be to use Bluetooth Low Energy.  If you want to use it in an ultra low power way, you would probably want to do some unusual things.  For example, you could wake up on key press, and then send a unidirectional broadcast to inform custom receiver software of the event.  If you want something that retries until confirmed, you could then have PC software briefly connect to the device as confirmation, allowing the device to stop broadcasting and go back to sleep.

Or you can be (or buy) a more typical bluetooth or proprietary wireless keyboard - but there's reason to believe this may consume more power than you were looking to expend.  Still, something like an add on numeric keypad might be about right for your need, especially if you rig PC software to uniquely recognize it and act on those keys in a custom way.