It happens fairly frequently that I'll finish up a big project or get done with a meeting around 30-45 minutes before my time to go home. There are usually things that need to be done, but it usually isn't worth it to start on those projects with only 30-45 minutes to go.
What are some good things to spend that time doing?
Go through your GTD system, sorting out of your inbox and updating your "someday" bucket.
Or just grab something small out of your "someday" bucket.