As far as Cardassian space, I like the assignments for unlocking _____ Officer Exchanges. They give you a variety of CXP (with a nice chunk of Diplomacy) and , ultimately, a shot at upgrading greens without spending dilithium. Colonization arcs are handy for similar reasons.
As far as Diplomacy itself, though, just parking around the Sirius sector block and doing the Receive _____ Dignitaries is always my preferred method; you'll probably be there anyway now and again picking up commodities for the Reputation grind. Plus, you can queue up a bunch all at once.