while 9 days could be a little on the long side, there is definitely a lot to see/do in pek (certainly more than the 5 days many tourists allocate).
for starters, i would go to one of the better great wall sites -- simatai or huanghua -- the latter only if you are in very good shape and don't panic easily.
you could also spend a day doing the summer palace, xiangshan (fragrent hills), and maybe the ming tombs.
i gotta make a break for brunch now, but i'll try to write more later.
in short, given the choice between extra time in pek and xian, i'd choose the former any day.