weights won't hurt your knees if you have the right technique and weight -in fact, they may even help by building up the surrounding muscles (going to the gym twice a week has kept my knee pretty much pain free, and i know this also goes for my trainer, who has had ops to remove cartiledge (sp))
thats a long time to spend on cv at the gym = hot, sweaty and dull. i would mix it up with weights, but if you aren't that keen on them, i would do intervals on each i.e 15mins on the rower, the x trainer at level xx for 1 min, and then level xx for 2, back to one etc, then 15mins on the treadmill and so on. will keep the boredom down.
what about classes i.e spinning? very good and last for 45mins.