Full disk encryption, that's it. When I get up, I lock the screen.
Just configure the top right corner to lock the computer (AFAIK it’s new in Mojave) or start the screensaver with an n-second delay for password prompt (configured separately under the screensaver tab). The delay is important because you will trigger it by mistake many times. The new lock option does not have a delay, which makes it a little less convenient.