Tournaments: Bet heavily on yourself, and win. Easy 4k denars a pop. Great in peacetime.
Beating people and taking their stuff: Attack enemy lords. Take their stuff. Caravans are fairly lucrative, but those spears mean you can die quick.
Less talking, more rrrraiding: Sea raiders have shiploads of good loot. Equip companions/yourself with the best, sell the rest.
Local knowledge: Buy trade goods where they're cheap. Take them to where they're expensive and sell them.
Slavery for fun and profit: Bludgeon people with a hammer. Take them prisoner. Sell them to their families. Stun enemy lords with sharp things, stash them in your dungeon, and sell them back to their faction for thousands a pop.
If you're at war, making money is easy. If peace breaks out, you may end up chasing the tournament circuit laden with trade goods, waiting for a war.