Rendell is a plausible choice. But if Kerry really, really wants to be President (and he really, really, really, really does), I think he's going to have to roll the dice on the Vice-Presidency. Other people have already suggested John McCain, and I think it would be a brilliant choice for Kerry.
Naming McCain as his VP would add more "Vietnam hero" credentials and create the image of a serious foreign policy team which could be counted on to pursue the War On Terror. It would also be a "Fusion ticket" which could appeal to Independents and moderate Republicans, and would reassure many swing voters after Kerry's heavy left-wing trip through the primaries. The liberal media, which has always loved McCain, would completely swoon over this ticket.
What's the downside for Kerry? Obviously some Democrats would choke at the prospect of putting a pro-war Republican on the ticket. But for the far-left this is an "Anybody But Bush" crusade, and they'll recognize and ultimately accept the political calculus involved. Yes, Ralph Nader will pick up some anti-war votes, but that will be heavily outweighed by all the swing votes that Kerry/McCain will put into play.
Would McCain do it? Why wouldn't he? He has no love for Bush, and this would be his only remaining chance to become President. (Remember, any Vice-President has good chance of eventually ascending to the Presidency. Five of the eleven post-WWII Presidents were former Vice-Presidents.)
Even if you go with McCain's ratings just over the last 5 years, he is much closer in votes to the 'perfect conservative' than he is to Kerry.
Remember, McCain has already been out campaigning for Bush and against the Democrats. The Democrats love to think that he is one of them, and he does play to them at times, and many conservatives look at him like he is one of them because of it. But he is not one of them, never was, and never will be.