With my son, we continued a ton of the stuff that he was interested in as a child.
Lego's, puzzles, etc...
It wasn't easy, if I asked him, he would usually say no, but if I was already involved in the project, he often would join in.
We also watched some TV together, and yes, I have seen way too many shows I didn't want to see, but if I found something interesting, I would ask him to sit with me, sometimes he would sometimes he wouldn't.
I found that making plans for myself (even if the plans were more geared toward him) and inviting him to join, but doing it either way, was a way to get him involved. If I tried to make plans with him, it didn't work so well.
I do the same with my BF's kids and it seems to be working a bit. Although we are forging different relationships...
I want to go horseback riding and while I got "maybe I will ride" from one and "ehhh" from the other, they will probably go and I won't be surprised if one of them ends up loving it.
"Acceptance doesn't mean resignation. It means understanding that something is what it is and there's got to be a way through it."--Michael J. Fox