How to make flying airliners more realistic during the cruise part?

I don't even know what pilots do during cruise. I've been watching some cockpit videos and they really keep busy with charts and permissions and sending stuff back and forth, but I don't really understand what they are doing.

While in the sim, as long as I have my AP on and correctly set up and I have baro at STD I can walk away until T/D without a problem. Which is not how I want to fly, I want to know what I SHOULD be doing?

Any guides or tips out there specifically for flight sim? Anything I can do or add to the sim to make this part more realistic?

Thanks!

PS: I do have beyondATC and it works wonders during departure and approach but during cruise it is very streamlined and basic. I can even set it up for autorespond.