If you looking to get some cash quick and you happen to be playing a catholic nation, then going on a crusade is a good option (should also work with Jihads if you are Muslim).Assemble 3 or 4 decent armies and ask the Pope for a crusade to holy land. While your armies are on a crusade you don't pay the upkeep, so that is the first bonus. Secondly, starting at Byzantium pillage every city on your way to the holy land, but don't leave your armies there. Destroy all buildings (gets you cash) and move on. You can give the city as a gift to one of your allies and castles to the pope to keep him happy (we want pope to be our buddy, plus castles would give too much advantage to our neighbours who can become our enemies soon). But I digress.
The reason you don't want to stay in the city for too long is that it takes some effort to keep the city happy and productive far away from your capital, plus your crusades will desert if you don't keep moving forward. You will loose some soldiers on the way, so at the end you may need to consolidate your armies, but with 3-4 stacks you will crush most enemies on your way.
Once you reach the target city you will be nicely rewarded with experience for your general and the troops. At that point you may start marching back and pillaging the same towns, that probably turned rebel by now (as I said it is hard to keep them happy...).
So, that's a quick and ruthless way to riches.