To enliven your fantasy RPG and drive character-driven motivation and adventuring, consider offering special experience point bonuses for character interests. Rather than a set amount (which would be diminishing returns as characters advance), these should be a significant fraction of what they need to get to the next class level, perhaps 5 or 10%.

Things like killing monsters, gaining treasure, finding magic items, turning undead, and learning new spells or weapons don’t count, because they are part of the usual process of gaining experience (and often are their own rewards, anyway).

However, you could ditch normal XP calculations and make all experience be gained thru these methods (sometimes called “diegetic advancement“) and therefore include such things. (More ideas.)

Either way, you might say the character only gets a bonus for the first three times before the thrill of a new tome or carousing wears off. You could also make it a requirement to achieve at least one of these bonuses in order to level up.

Note that any character can go carousing and earn advantages (and weird experiences), but certain characters can also sometimes get a significant XP bonus for it, because the discussions and contacts made (before things get too wild) can be very instructive.

Any

All characters involved gain extra experience points when they:

1. Earn an invitation and attend a fancy dress ball or the elevation of a new bishop/high priest, where they hobnob with nobles or church officials.
2. Help return a fallen party member’s remains and personal possessions to their family.
3. Help give a fallen party member an honorable funeral.

Fighters

You gain extra experience points when you:

1. Spend a week studying with a warrior at least 4 levels higher than you whom you’ve never worked with previously.
2. Spend a week studying a dead or captive monster you have never encountered before.
3. Go carousing in a new city.
4. Recover an ancient treasure and return it to its rightful place.
5. Defeat a worthy opponent in single combat.
6. Adventure in an exotic foreign land.
7. Advance your heroic duty from your character background.
8. Reconnect with old friends/family.

Paladins

You gain extra experience points when you:

1. Spend a week studying with a warrior at least 4 levels higher than you whom you’ve never worked with previously.
2. Spend a week communing with a holy site of your faith.
3. Fulfill the central tenet of your faith (destroy an evil idol, kill a monstrosity, etc.)
4. Do something self-sacrificing (morally right but not to your advantage).
5. Recover a relic of your faith and return it to its rightful place.
6. Defeat a menace in single combat.
7. Get a knighthood.
8. Advance your heroic duty from your character background.

Rangers

You gain extra experience points when you:

1. Spend a week studying with a warrior at least 4 levels higher than you whom you’ve never worked with previously.
2. Spend a week communing with a forest you’ve never experienced before.
3. Spend a week studying a live monster you have never encountered before.
4. Save (or significantly help) a large animal or group of small animals.
5. Defeat something that menaces your chosen wild land.
6. Do something self-sacrificing (morally right but not to your advantage).
7. Adventure in an exotic foreign land.
8. Advance your heroic duty from your character background.

Wizards (studious magic)

You gain extra experience points when you:

1. Spend a week studying with a wizard at least 4 levels higher than you whom you’ve never worked with previously.
2. Spend a week studying in a library you have never visited before.
3. Recover a rare tome.
4. Recover an arcane artifact and return it to its rightful place.
5. Defeat a more powerful wizard or sorcerer using magic.
6. Fulfill the requirements to be made a master of magical arts by the guild.
7. Adventure in an exotic foreign land.
8. Advance your heroic duty from your character background.

Sorcerers (innate magic)

You gain extra experience points when you:

1. Spend a week studying with a sorcerer at least 4 levels higher than you whom you’ve never worked with previously.
2. Spend a week communing with a place attuned to your source of power.
3. Recover an arcane artifact and return it to its rightful place.
4. Make contact with an otherworldly being of great power.
5. Defeat a more powerful wizard or sorcerer using magic.
6. Learn an ancient truth about the realm.
7. Adventure in an exotic foreign land.
8. Advance your heroic duty from your character background.

Clerics (divine magic)

You gain extra experience points when you:

1. Fulfill the central tenet of your faith (destroy an evil idol, kill a monstrosity, etc.)
2. Spend a week studying with a cleric of your faith at least 4 levels higher than you whom you’ve never worked with previously.
3. Spend a week communing with a holy site of your faith you have never visited before.
4. End a threat to a holy place or return a relic of your faith to its rightful place.
5. Do something self-sacrificing (morally right but not to your advantage).
6. Spare one or more opponents and convert them to your faith.
7. Advance your heroic duty from your character background.
8. Reconnect with old friends/family.

Druids (nature magic)

You gain extra experience points when you:

1. Save (or significantly help) a large animal or group of small animals.
2. Spend a week studying with a cleric of your faith at least 4 levels higher than you whom you’ve never worked with previously.
3. Spend a week communing with a forest you’ve never experienced before.
4. End a threat to a holy place or return a relic of your faith to its rightful place.
5. Do something self-sacrificing (morally right but not to your advantage).
6. Learn an ancient truth about the realm.
7. Adventure in an exotic foreign land.
8. Advance your heroic duty from your character background.

Rogues

You gain extra experience points when you:

1. Spend a week studying with a rogue at least 4 levels higher than you whom you’ve never worked with previously.
2. Go carousing in a new city.
3. Make shady/influential contacts in a new location.
4. Recover a rare treasure and consult an expert about it.
5. Learn a dark secret about the realm.
6. Adventure in an exotic foreign land.
7. Advance your heroic duty from your character background.
8. Reconnect with old friends/family.

Unusual Species

You may also want to allow characters of unusual species an experience bonus for making connections or spending time with those of the same species, as long as that species is a minority in the culture. This can feel punitive to players whose characters are of the common species, so it may be best to only allow such a bonus in the absence of other bonuses for class activities. For example, perhaps the human cleric goes off to the local temple to spend time with a more powerful cleric while the tabaxi rogue spends time with a group of tabaxi. The tabaxi rogue should not also be allow to get an experience bonus for carousing, whether it’s with the tabaxi or other characters.

You may instead choose to even things out by allowing any character a bonus for spending time with any unusual (friendly) species. A human could learn a lot from some tabaxi, after all.