I never seem to get bored even on some routes I've done hundreds of times.
I just fully engage all my senses and try to soak in the entire experience. There is always something different. Sights, smells, noises, etc.
I also interact ever so slightly with people I encounter. Bell, waving hi, saying howdy.
I also do each ride with a purpose and pay attention to what I'm doing so I can go faster with less effort.