What can I build my engineering boyfriend for the holidays?

Since you can already solder and mess around with an ESP32, I'd build him a desk gadget he can keep hacking on. A macro keypad is a solid one. A few keys, maybe a little rotary encoder for volume, an OLED screen showing whatever he wants. You do the hardware and the base firmware, and he can rewire the key functions forever. Engineers who code love that kind of thing because the project is never actually finished, wich is the point.

I reckon he might prefer somthing that shows him real data instead of just a macro pad. Since you're allready thinking about those microcontrollers, you could set up a cheap ambient temprature sensor too. Engineers often get into IoT things where they're monitoring workshop conditions. A little OLED displaying the local hum of the power grid or the room temperature might be more useful than just a key remapper. He'd probaly want to see the data stream live.

I think he might acutally like something that tracks power consumption. Instead of just displaying temperature, you could wire up a small current sensor to the computer's power supply input. It gives him a live feed of how much wattage his rig is pulling when he's running simulations. That's actual useful data for an engineer who cares about efficiency. It's cool to see the numbers change minute by minute when the compile finishes. Even if it's just a small setup, seeing the real usage stats on a tiny screen is a solid gift.

I am thinking about something that just tracks when he is using his station. Like a little simple counter that keeps track of how many hours he has been working at the desk in a day. It would be a cool way to see his work habits without being annoying. It would be a project that is just about simple counting and showing the result on a small screen. Does he ever say he wants a way to see how much time he spends on his projects?