Things to see by airport.

The usual duty free stores, airline lounges and restaurants you'll find in any other airports.

If I exit the airport on a 72 hours free transit what is close by of interest?

To be honest, a six-hour layover is way too short to do much sightseeing. There is nothing of interest around the airport at all. Clearing immigration and taking the airport express to the outskirt of downtown takes about an hour, then from there you need to get around by subway. I guess you may have enough time to check out the (in)famous Tian'anmen Square and the Forbidden Palace, or the gigantic shopping malls next to the downtown airport express stations. Make sure you leave for the airport at least two hours ahead of your flight departure time so you'll have enough time to clear security and immigration.