Be proactive.

I make plans to do other things I enjoy. Computer time is a filler if I'm not busy doing something else, like spending time with family/friends, being outdoors, running etc.

I agree with mbrock also that avoiding a smartphone (or having a data-less plan) is a good way to stop time being swallowed up. When you're on a net-connected machine eight hours a day at work, you don't need more screen time!