What activity or interaction particularly makes you feel "alive"?
For some people it's singing or dancing, others it may be getting lost in a book. Maybe skateboarding or creating art or running. Teaching. If I had to pick an emphasis, it would be what things you do with other people - teaching, watching a movie together with friends, whatever. But it can be anything.
I implore you to do more than just pick something you "like doing". For example, I have enjoyed watching a movie, teaching someone something new, flirting with girls/etc, playing videogames. But some things that really make me feel alive, and have a certain energy to them all of their own - those things are separate from just "things I like to do".
One of them is playing something like Frisbee. Or even just the act of throwing and running one down and catching it. It's like a dog playing fetch I suppose. It's therapeutic for me, to just run, and do. I've liked other sports things, but, the nature of doing that is all its own.
Another is, actually, running a meeting. Very strange. But, provided people actually want to do something, it's one of the most exhilarating and personally rewarding things I do. Running / facilitating a meeting where people are deciding things and making decisions, and then actually carrying out those projects and executing them - this is something that deeply makes me feel alive. Sometimes I have to restrain or contain myself, especially if I'm not actually "in charge" (I had this happen tonight). Seeing "the troops" there, people working on different projects, little side meetings, people wanting to contribute and move towards their goals, it is really a great thing. I realize a large part of it is having good people to work with / people who actually want to do work. But even without that, I still like facilitating such things.
Those are a few things, I may add more later.