We're doing something similar so I spend a lot of my time in the library reading stuff like Lonely Planet and the Rough Guide, and looking at sites such as virtualtourist.com.
We generally do our own thing and usually manage ok. There's always touts waiting to greet the ships, usually offering tours similar to the ships. If you know the shore excursion prices you'll know if you're being offered a good deal - and you should always bargain.
wikitravel isn't a bad site for giving you ideas of what to do.
Bear in mind that for Beijing, you'll probably be docking miles and miles and miles and miles away from Beijing (as will we but we have an overnight stop there) so don't completely dismiss the shore excursions. Convenience can be worth paying for.