You need to think about what you like to do. Are you an artist? More of a crafter (like to do things with your hands)? Do you love gardens? Would you love to write if you had the time? Music? Cooking? Baking?
Good for you for joining a gym! You can work on your health while you have a good time. And a little exercise can brighten your day. It's some sort of chemical thing.
Anyhow, once something grabs you, you see if and where you can fit it in to your life. Some things you'll need to classify as "wait for another season."
When my children were young, I did a lot of needlework because it was easy to set down and pick up - and I could take it places with me. I would also have knitted or crocheted if I'd known how.