This is a real issue, and it is something I wish I had spent more thought preparing for. 100 hours a week of free time is a freaking boatload of free time. I have started some volunteer activities, new hobbies, and will be a census taker. I still need a long term passion project. It is too easy to fall into old activities and do them too much.
I did some reading before FIRE, so I knew I needed to have more planned activities. For me, getting bored gave me the motivation to go out and start trying some new things.