I often struggle to come up with ideas of things to do with my friends and significant other. This is in spite of the fact that I generally don't struggle to do interesting things in general, and never seem to struggle to find interesting topics of conversation.
I've come to realise why this is. The things I do on my own generally fall into one of a few categories: work-like things, hobbies which fully immerse me, or spontaneous things.
Work-like things are things which would be work if I didn't want to do them, was forced to do them, or had a schedule for doing them (these are not mutually exclusive categories). They include a lot of hobbyist research on things I'm interested in (a lot of which I have posted to LW). These are unsuitable for doing with other people almost by nature: if someone told me to do something, or if I felt I had an obligation, it would be a form of work. The same goes with me telling someone else to do it. Having another person involved also makes it much more complicated, which makes it more effort, which pushes it towards being work.
Hobbies which fully immerse me are generally my main way to decompress myself after periods of work. Typically this includes video games, TV, reading; also writing, playing and recording music. As I am at least mildly introverted they have the benefit of not including human interaction. They also allow me to occupy my brain with thought without involving the infinite stress of the real world. For the creative parts of these, I have realized that the process is the important part, not the result.
Spontaneous things are often things which my friends organize for me, but which I allow into my life frictionlessly by just saying yes to things. My own whims can also substitute for my friends in some cases. Typical examples: going to a university society meeting for which I had no previous interest, going on a long walk/bike ride.
There is clearly a large amount of overlap here. Most of my hobbies cover at least two, and often more, of these.
I am working towards a solution. It seems the solution is just practice. For many people, large parts of their lives are well-planned anyways. If you're very interested in things like museum or art exhibitions, then this is necessary. This means when they have to plan a group activity, they know how to find something and do it.
A few months ago I had no idea how to plan something like that. I struggled to explain it, but when I reached inside my head for the "plan an activity" it was like being asked "just lift your tail up". There was no visible path in my brain. With some advice and practice I am improving. I suspect things will get easier, if nothing else because I am building a repertoire of things to do (websites that have lots of hikes on them, known venues to visit with people, etc.).