Wow, you've been active.
It seems to me that running and tennis don't inspire you anymore. I encourage you to keep trying new stuff as you have been, but I will add that, rather than just randomly trying new stuff that seem interesting and seeing if it hits you, I suggest that you ask yourself some guiding questions so that you can get a better sense of what exercises provide that inspiration.
What movements do you already enjoy doing?
What movements would you enjoy doing that you haven't already considered?
What physical qualities, skills and abilities do you admire and would aspire to learn and master?
How do you want to feel both, during and after the exercise? (answer with specific feelings)
These kind of questions are good for getting a clearer guide when searching for a new activity that you plan on doing long term.
I haven't tried kickboxing but I'm a martial artist and I think it definitely the type of thing to try out. The beauty in all martial arts is the variety of movements, tempo and intentions in their practice. I struggle to see how anyone can get bored with a martial art, other than that they didn't stick with it long enough to let the form ingrain itself into their body. Once you get a feel of a specific system, your hooked.
Keep searching and good luck. I'm sure there is some exercises or sport inspiring enough for you to take up long term.