Nice strategies. I agree
To add some of my own stuff here...
I'm currently using many spies and assassins with my Egypt campaign. I've been at war with the Byzantines pretty long now but I've been hardly fighting military battles with them after some time. I just perfected my spies and assassins after long waiting to attack their settlements one by one starting from the nearest of course.
I basically make those settlements riot by sending in many spies and destroy the buildings that increase public order with my assassins. Buildings creating income are good targets too. I also assassinate any important characters with my super assassins who can kill most of them. At some point the settlement turns into a Rebel one and there ya have it! Heh heh. Gotten rid of some of the enemy power using patience and lots of sneakiness
At the moment with my Egypt play I've got a couple of super spies and assassins picking of any possible character and settlement all the way in EUROPE. Their royal bloodlines -- even neutral ones -- are decimated significantly. Worked well on Denmark 'cause they were on a Crusade against my Islam power, heh heh.
Man, I even assassinate allied factions, just for FUN!

For instance the Turks who are of course weakened a lot in the campaign are my nearby allies. To keep my faction's reputation high enough I didn't plan to conquer them by military force, but to use that same strategy of sneaking in spies and assassins to turn 'em into rebel settlements. Thing is there's hardly any effect . Too bad. I think I'll just try the plague technique on them.
In diplomacy when the enemy priority is Peace I try to demand as much money as possible. So if you propose Ceasefire and you receive a Very Generous notification I fill in a number of cash every time as high as possible and it has to be just below Balanced, so you'd get Generous. If you're lucky enough they'll be bankrupt. Then again, they usually are bankrupt or their cash is meagre, heh.
Well, that's all I have to add for now
