1. Take Safari and the App Store off your phone. If you have an iPhone, you can do that with Restrictions.
2. Move everything off your home screen. Minimise the icons on your Dock. (Mine has Messages and OmniFocus.)
3. This sounds crazy, and probably isn’t necessary, but to make your phone boring - make it greyscale! Accessibility, Display Accommodations, Colour Filters.
I look at my phone waaaaay less than I used to because it simply doesn’t do anything. It should go without saying that Twitter isn’t on there. I don’t use Facebook.
My leisure time device - the one I’m on now - is my iPad. It’s a conscious choice to use this device; when I pick it up, I know I’m “just browsing crap”.