Get some merchants to the Ivory, spice and especially GOLD deposits in Africa. There are (as i remember) 2 gold + 1 Ivory in west, in what would today be the border of Morocco and Algeria (you can just see one of the goldmines on the brim of the blackened map-part). There is some Ivory and lost of spices (and other middle income resources on the west bank of the Red Sea (Egypt).

The gold mine easily generate 1000+ each pr turn (finance of 3-4). Last i check I got 1500+ with an 7-8 finance merchant on a goldmine with my capital in London (profits are positively affected by distance to capital). And when merchants are so far abroad, they develop trait fairly fast. My experience is that you have to fight some other merchants in Egypt, but in the west you have no competition.

Of cause, as someone else mentioned, you can always go on a sack'ing rampage. It doesn't need to be a crusade (which has the positive of cheap crusade armies and no upkeep, but the downside of major side trips resulting in desertion).

Jason X makes a very good point, thou easily overlooked.
"chivalry of the governer (which also helps the population boom)"
I think you get 0.5% population growth for each chivalry, that give you 5% at max chivalry. That can make a Castle into a Citadel in no time which is a life saver in hard campaigns / hard difficulties. It's also good for buffing up cities which have build what is needed on the current level, but still needs some population to upgrade.