This is great advice, and it's what worked for me. For a while I was terrified of just going to the store or walking down the street, all I would do is stay in all day. Now I make sure to be out in the world every single day for at least an hour.
It's all about desensitizing yourself by very slowly exposing yourself to your fears. It's that simple. I started with small things, like taking a short walk. Then I'd walk to the store to get a few things. If I had a panic attack...I'd recuperate and wait a day or two before trying again...but you always have to try again.
One of my favorite free activities is just taking walks, admiring the city...maybe bring along an mp3 player and listen to some music or you can listen to an audiobook. If you have a local park to walk around it's even better. Even when I don't have a lot of money I still like to pop by the thrift store and see what's new. Sorry, I'm easily amused and dull, that's about the extent of the things I do for free. I'm sure if you wanted to you could find local activities through Google.
The only thing you need to do is something instead of nothing.