Spending money to influence allegiance -- with effects that should probably not be guaranteed -- might be interesting. Very Cold War-ish. Money for intelligence (fleet dispositions, economic data... communications intercepts and forgery
) might also be interesting.
It can be taken too far (SE:IV was a bit extreme in that a strong espionage lead could be used to achieve the defections of fully-functioning warships, even while they were in transit, turn after turn; likewise, it was probably too easy for spies to send a declaration of war and blast a trade/research treaty into nothingness -- bad since those took time to ramp up), but warfare through other means sounds tasty.