Like many said, trade is a good option http://forums.taleworlds.com/index.php/topic,46386.0.html . Swords, armors, shields etc, will also sell for more the higher your trade skill but, unlike trade goods, the place where you buy and sell it won't have any effect.
If you bet 100 denars at each round in a tournament, you may earn about 4k when you win. If you hate the possibility of having to fight with a bow, you can go here http://forums.taleworlds.com/index.php/topic,46290.0.html and tweak it for your needs. Or just choose the locations where it won't happen. This list might contain some errors, I made it sometime ago.
Vaegir:
Khudan - everybody mounted - lance/shield, bow and arrow, javelin/shield
Reyvadin - mounted and foot - lance/shield, sword/shield, two handed sword, axe/shield
Rivacheg - mounted and foot - lance/shield, bow and arrow, sword/shield, two handed sword, axe/shield
Curaw - everybody mounted - sword/shield, bow and arrow, javelin/shield, two handed sword
Swadia
Uxkhal - everybody mounted - sword/shield, two handed sword
Dhirim - mounted and foot - lance/shield, bow and arrow, axe/shield, sword/shield, two handed sword,
Praven - everybody mounted - lance/shield
Suno - mounted and foot - lance/shield, axe/shield, sword/shield, two handed sword,
Rhodock
Yalen - mounted and foot - bow and arrow, lance/shield, crossbow/sword, sword/shield, two handed sword
Veluca - mounted and foot - bow and arrow, lance/shield, crossbow/sword, sword/shield, two handed sword
Jelkala - mounted and foot - bow and arrow, lance/shield, crossbow/sword, sword/shield, two handed sword
Khergit
Halmar - everybody mounted - lance/shield, bow and arrow, javelin shield, sword/shield, two handed sword
Narra - everybody mounted - lance/shield, bow and arrow, javelin shield, sword/shield
Tulga - everybody mounted - bow and arrow, javelin shield
Ichamur - everybody mounted - bow and arrow, javelin shield
Nord
Sargoth - everybody on foot - sword/shield, two handed sword, axe/shield
Tihr - everybody on foot - sword/shield, two handed sword, axe/shield, bow and arrow
Wercheg - mounted and foot - axe/shield
Selling prisoners is alright the way it is, in my opinion. If you think 50 denars isn't worth for a knight, there's a tweak to make Ransom Brokers pay according to your prisoner's level. Capturing lords will give you a good ransom but it might take sometime before somebody decides to pay for it.
Like MountainBlade said, there's a peace quest with a reward of 12000. You won't receive all that ammount though, as you need to bribe the lords and, unless you have a persuasion of 10, is better to just pay them right away. Usually the first one you contact asks for 7000, the second asks for less, about 2000 or so.
If you'd rather make your wealth by sword, Sea Raiders are the ones who give the best loot and best XP. Raiding caravans is good too, they give you free food and some trade goods. If you don't want to attack them, you're still able to tax them and earn a few denars. Raiding villages and forcing peasants to give you supplies or cattle is good too.
Cattle. There's a quest you can get from guildmasters that asks you to escort cattle. Many people do a little trick, they wait a whole month propositally near the cattle so the quest fails and you have the cattle for free. Seeing as these quests give about 100 cattle, you can make large sums by selling beef. Don't slaughter the animals far away from a city though, as beef gets rotten in about 1 day. Some villages sell cattle for 22 denars, each cattle makes 2 beefs. Also, you can force peasants to give you their cattle.
If you wait inside a town near payday, your troops wage will be cut in half. Disband any mercenaries you have in your troops, they cost a lot of money.