Before every flight, I turn on all of the failure options and set the time for the failure for each of them to something like 1-1000. This means that I do not get a failure during every flight, but I will get one from time to time. Doing this forces me to monitor all of the aircraft systems throughout the entire flight.
In addition to monitoring for failures, I’m also always keeping several steps ahead of the aircraft. This includes setting up expected radio frequencies, setting up my vhf nav radios (for backup even if I’m using GPS) to verify my location, reviewing my next fix, reviewing the anticipated arrival and approaches. Reviewing the arrival airport map to determine what taxiway I believe that I will turn on and which direction that should be. Reviewing the taxi routing that I will probably been given to my parking spot so that I’m familiar when I arrive. Oh, and never forgetting to watch for those failures.
There is a lot that should be done while en-route and doing these things should keep anyone busy. Anyway, that is what I do during the cruise phase of my flights.