Let me state first and foremost, I don’t have a clue how many “astronauts” there are on Earth these days. Not everyone of them can be a Neil Armstrong or a Buzz Aldrin. Now that the use of the shuttles have made trips to the space station or repairing the Hubble and satelites almost routine, the astronaut has fallen through the cracks.
The average person doesn’t seem to realize (at least in the U.S.) what the selection process is for these individuals. They are pilots that are pulled from the various branches of the military. They have exemplary records and are in peak physical fitness. With all the jobs that are done in space, now they may not even be pilots, you need engineers, mechanics and computer experts. They do an amazing and incomprehensibly dangerous job. It only comes to the public’s attention when there is a disaster such as a shuttle exploding or something similar.
As far a jobs when they are grounded, or back on Terra Firma for good, there are many options for employment. These jobs may not have all the glory of going to the moon, but they are still important.
One is drumming up support for future missions. If a “real live” astronaut came to my school as a kid, it may have made me want to pursue that line of work. Having these “spacemen” talking to Congress could help drum up more dollars for missions to other planets or research. The same would go for private corporations. We still have no defense against near Earth objects. An astronaut has real space time which could put a realistic view on plans that are just on paper.
The most important job I think that they could fill would be in position of trainers. They have been in the situations that the textbooks are trying to explain to the new recruits. Like in the military, combat experience will make you a better instructor than someone who has studied books for a lifetime. These professionals have experienced what it is like to lose all power when you are orbiting 200 miles above the surface. More importantly, the fact that they are teaching the class, means they survived it. They have the real world (or out of this world) experience to teach the “new guys” what can happen and how to fix it.
There are other things that astronauts do, politics and such, but they should use their one of a kind skills and prepare the next generation for space. We are going farther and farther, which means we will need more and more of these highly skilled individuals. Who better to train them than the people who have been there themselves?