In my current campaign I'm presenting the players with multiple ways to earn their characters some experience points (which I'll being calling exp hereafter).
up to 100 exp per Level or HitDice of foes defeated (not necessarily slain)
200 exp per level rating for a missions/quest successfully completed.
PCs may spend funds to gain experience by various means:
1. Training: Each week of downtime the PC may spend 1 groat per exp up to 100 x level.
2. Carousing: Each week of downtime spent carousing will earn 1 exp per groat spent and possibly more but there are hazards to carousing requiring a weekly check to endure.
3. Magical Dabbling: Each week instead of simple training a magic-user can spend resources and time on bizarre endeavors at up to 200 groats per level per week. Training uses up time that could be spent on dabbling.
4. Sacrifice: Each week sacrifices may be made to the appropriate deities. A week with carousing generally doesn’t allow one to also make sacrifices. up to 300 groats per level per week may be spent on sacrificing but the sacrifice is only rewarded on a successful piety check.
5. Greasing Palms: Thieves may spend money on bribes and fostering connections at up to 200 groats per level per week for exp.
6. Tribute: Fighters may send tribute to sworn overlords at unto 500 exp per level a week. No time for carousing when this is done. If the gift is not well received the groats are lost and no exp are gained.
Some activities and behaviors can also offer exp awards or modify other ones earned:
1. Leave your mark on the Walls of Fomorgard, The White Way, and The Mud Run: 150 exp each.
2. Traveling over, under to through the HedgeWall : 100 exp each up to 5 times until level 4. (no zigzags, must be true trips)
3. A Fighter who slays in single combat another warrior of monster of 4HD or more the first time earns a bonus of 50% the exp.
4. A Fighter that survives in 3 charges in a major battle earns 500 exp
5. A Fighter who trains with or survives a challenge with 3 different Weaponmasters earns 1,000 exp
6. A fighter who challenges and survives against all worthy foes on a road, at a crossing, or a dungeon stair for a week earns three times the normal EXP amount for each foe defeated during this time. May be done no more in their lifetime then a character has CHA points.
7. Building a castle or taking an occupied castle earns the master of the Castle 1% of the value of the place in EXP.
8. Shaking the hand of the Mayor of Toadtown (or picking his pockets) earns a thief 500 exp (first time only)
9. Slaying a traitor earns a thief x5 exp the first time they do so
10. A thief establishing a gang of 12 or more thieves earns 2,000 exp
11. A thief may stealing a single gem worth 10,000 groats or more from the trove of a dragon, the vault of a king, or the lair of a giant earns the thief the full exp value in groats without having to spend the wealth but only if that is the only item stolen this mission and the thief leaves a calling-card/message.
12. Mixing 2 different potions in your stomach earns a Magic-User 300 exp, may only be done as many times as the MU has points of CON
13. Learning or Stealing spells from 8 hostile Magic-Users earns a MU 4,000 exp
14. Visiting 6 power points will earn a MU 5,000 exp. (only the first 6 count)
15. Binding a powerful supernatural creature into a place or in an item earns twice as many EXP as defeating the creature otherwise would.
16. Defeating, with magic alone, 3 supernatural foes of 10 HD or more earns a one time reward of 5,000 exp
17. Securing or Manufacturing 3 Artifacts earns a Magic-user 50,000 exp.
18. Earn a key to the The Visitors Ward of Gnomeville and earn 500 exp
19. Win an Audience with the Oat King and earn 500 exp (for first 3 visits)
20. Kiss the hand of the Ice Queen and survive to earn 600 exp (for first 3 such kisses)
21. Navigate 30+ miles of the lost river and earn 500 exp
The 21 activities above and the 6 methods to convert cash into exp are to answer "So, what does my character do?" some are class oriented and can't help everyone and a few are really only attainable by one class but others can give it a shot.
Some of the ideas were inspired form this post at Sham's grog and blog but the nature of my campaign and rules differ just enough I ended up assigning exp awards for various tasks instead of otherwise going with the pretty interesting set of ideas there.
I hope to have 3 weeks down time between sessions now and again if not regularly so players can assign downtime activities to the PCs, but the action got paused in the middle of a spot in the dungeon where I couldn't pull off- "and 3 weeks later".