I have used this technique a few times and it always has good results. I write down a list all the things I do most in my free time. It may take a day or two you can keep a journal with the times you spend on each activity. I then remove the top 3 activities from my life for a week. These generally tend to be the most unproductive things I do in life.
Beforehand I have no plans on what I'm going to fill up my extra time. I just make sure I don't do the things I usually do. Every time I've done this I find something more fun, exciting, and rewarding to do with my time.
Many times I will get stuff done around the house I've been putting off but more importantly I tend to pick up new hobbies that I enjoy from then on.