Smart plugs are the most useful cheap home-tech upgrade. The Kasa Mini is the one I'd recommend to everyone because it's small enough to not block the second outlet in a duplex receptacle, reliable, works with Alexa/Google/Apple Home, and costs under $10 each in a 4-pack.
My favorite use: remote router reboot. Plug your router into a Kasa, and when your internet goes sideways from a coffee shop 200 miles away, you can remotely power-cycle it from your phone. 95% of ISP issues clear up this way. Beats begging a housesitter to find the power cord.
Also good for: holiday lights on a schedule, space heater auto-off at bedtime, bedroom lamps for kids who can't reach the switch, and letting Alexa turn off the coffee maker when you forgot and left the house.