I have a 5 hour layover on my return flight to the US in Frankfurt. I will just have a carry-on bag. In that time is there anything I could do in/around the airport, or should I just suck it up and read a book?
That Frankfurt airport guide isn't very helpful if you just want to quickly run into the city. It is fine if all you want to do is stay at the airport, but not if you want to come to Frankfurt.
Rather than type this information all out, have a look at this Trip Advisor post that tells you exactly how to take a train into Frankfurt. Plus, things you can see and do there.
5 hours is not a lot of time, and for flights to the US, you will want to be walking back through security about 2 hours before departure. My best guesstimate is that you are only going to have an hour or so in the city. You could cut the time waiting for a train and spring for a taxi to take you to the Römer, and then another one back, but you are looking at 30-35 € each way. That would give you closer to 2 hours. If your flight is delayed arriving, just call off your excursion and plan for a longer layover next time.