(1) Propose that the United States unilaterally offer to reopen its embassy in Tehran. Ditto for Cuba and North Korea (and Bhutan, I suppose, though I don't really know what the deal is with them). Make the point that we live in dangerous times, and diplomatic relations should be used as a way of more effectively dealing with the world, not as a way of making self-righteous statements of approval or disapproval about specific regimes.
(2) Propose a specific list of Bush administration executive orders that he would rescind. No shilly-shallying, just a flat promise to revoke them. Possibilities include the orders governing torture, military commissions, and FISA. If he wanted to be even bolder, he could categorically promise to halt the use of presidential signing statements.
Like I said, I don't really know if either of these things would resonate strongly (and I don't know if Obama thinks they're good ideas, anyway). But they're simple, they appeal to the same instinct that Obama is appealing to in his campaign, and Hillary would oppose them. Worth a try?
POSTSCRIPT: Obviously, an unqualified promise to pull all troops out of Iraq and shut down all American bases by, say, January 20, 2010, would also be big, bold, and something Hillary would oppose as naive. Unfortunately, Obama has already taken a stand on this issue. A sudden spine stiffening on something he's been asked about hundreds of times before would probably generate as many questions as it would answer.