Make the very most of it. You may never say this again in your career. The more common question is "what happened to my spare time?"
When I say "Make the most of it." I do not mean "lie back and enjoy it." I mean use it very wisely. Learn, work on a personal project, join an open-source project, do ANYTHING with that time which will advance your skills (both for you and the company you work for).
The one time in my life where I was short of things to do (I was key to one project which meant I needed to be free on demand, so I couldn't really join other projects properly), I actually convinced the company to pay for a metric ton of O'Reilly books and sat learning all about web development. Which didn't actually come in handy til some years later but, when it did, it really did.
Edit: That was obviously back in the days when we had to learn things from books. You could get an O'Reilly subscription nowadays, or just cruise the internet, or read Stack Exchange sites all day.