In my experience, the only mutually interesting topic is for them to spend time with their parent. If whatever you're doing is interesting to them and you spend time with them doing it, they'll remember it for the rest of their lives.
I helped one of my sons learn some basic electronics and he showed a lot of skill with it, but he soon lost interest when I tried to turn it over to him to explore. He didn't seem interested in pursuing the topic as much as spend time with me. Nowadays it's mostly talking about the software projects he's implementing and how best to architect or design his code. He gets annoyed when I become too pedantic, so mostly it's asking questions and validating (and guiding) his choices.
I've had a challenge doing activities with my oldest, as our interests hardly match up, and I wasn't able to keep the interest (I'm stubborn and self-centered). Mostly I have to listen--I'm not sure whether he enjoys me listening, but he keeps doing it when the opportunity presents itself.
My youngest--I'm at a loss. He's high-functioning autistic and plays games most of the time. He's smart and witty, so he enjoys the nuances of conversation and actually doing things with the family. He often doesn't care what we're doing, just that we're doing it together.
We do some activities (plays, musical performances, etc.) as a family (most of the time minus the eldest, as he works, etc.), and they remember and appreciate and are fond of those times when they think on them. It may not be that they were necessarily interested in them, but the fact we enjoyed the time together is what counts.
Which is to say that trying to find a mutually interesting topic is futile--they all have their distinct interests and you can't change that. But what you can do is spend time with them, pursuing their interests. It takes a ton of energy and effort, but they will appreciate it and hopefully take the same habits into their own families.