very light bdsm, you can start reading some books. Please, whatever you start, bear in mind that it has to start very slowly and lightly. Kink in general destroys monotony, and there are a lot of kinks. You can also go to some clubs, make exchanges, start looking into fetishes...
If the issue is serious, go to a consultant, it will help a lot.
Since you respected the rules, I'll give you my personal experience.
I'm a man, been with my wife for almost 14 years, no children in common.
We started facing boredom, sex was less frecuent and overall worse. It was pretty obvious since we usually had 1 to 2 moments of sex each and everyday. Since I did some BDSM with anterior love relationships, I offered her some experimentation, but she was reluctant. After a serious crisis, were she almost hit me while arguing (I'm a very calm usually), I locked myself in the bedroom and we both started crying. The next day we both recognized the problem, and went to a psychologist. The psychologist didn't actually help us, but rather reommended us to a "consultant of couples". We agreed.
That consultant was a very good idea. The first times we talked a lot about actual problems, and identified problems that didn't even consider as problems, and we realized how much we had to fix; next we made exercices to build confidence, trust, intimacy... and then sex started to be better and more common. A critical point here was space, one of the things that ended our good relationship was the imposal of burden into the private space of the other (myself or her), what we did was to make a certain, common couple space, and to respect the private, individual, space of each one. This was critical, the moment she, or me, stopped depending on each other, we had a better relationship. Lastly, we explored into BDSM, I propossed to be the Dom, as that's what I know and what I identify as, and we both agreed. I spent some months remembering the ropes, bought some fresh equipment... CONT...