From the game's encyclopedia
An activity where a character chooses to travel to a famous monument or landmark to improve themselves, by delving into its history, knowledge, and mysteries.
Anticipated rewards:
• Intrigue SkillDiplomacy SkillMartial SkillLearning SkillStewardship Skill Skill Points
• Court PositionExcellent-Aptitude Court Positions
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Monument Expedition is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "An activity where a character chooses to travel to a famous monument or landmark to improve themselves, by delving into its history, knowledge, and mysteries." It is part of Activity. Patch 1.20.1.2, from game/common/game_concepts/01_bp3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/01_bp3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.