Completing the Devil by the Well contract in White Orchard will give such a trophy, one of the earliest available in the game. Killing most of the game's major monsters and bosses will give you a trophy that you can display on Roach's saddle to gain various bonuses, some of which confer additional experience.
These always offer bonuses to experience gained from killing enemies, sometimes as much as 20%. These weapons can be found across the game with numerous variations and upgrades, identifiable by the green text in their name. That being said, there are numerous actions you can take to speed up how quickly you gain experience along the way.
