Old Man's War is one of my favorites of his, and if you like it, there are several more written in that universe. (Skip The Sagan Diaries, though.)

For standalones, Agent to the Stars and The Android's Dream are both fun reads. If you like metacommentary/community reads, Mark Oshiro over at MarkWatches/MarkReads just started The Android's Dream, so you could read along if you wanted to.

If you like the above, under no circumstances read The God Engines. He wrote it deliberately to be as different from his other stuff as possible. If the above books don't do it for you, maybe give TGE a try. (I hated it, and I've liked most of Scalzi's stuff.)

You can read some of his stuff free online at http://whatever.scalzi.com/about/the-scalzi-creative-sampler/