I sort of enjoy the process of leveling up a character. More so than creating the character in the first place. In newer editions of the game, leveling can take awhile (pathfinder, D&D 3,4,5) however it can still be a bit of fun if your a math nerd. Some of these systems you start to get into real feature creep however.
That all being said, leveling in OSR, and older D&D type games is fairly quick and easy. Get a few extra spells, add some extra hit points and viola new level! It's wonderful that it's so fast to do (part of the charm of old school gaming). But it can take a really long time to move between levels, and the pay off isn't always that great. So rather than being a little down that you only get a few extra's, here's a few ideas for old school leveling.
1. Increase an attribute.
Upon leveling up can attempt to increase an attribute score. The player decides which score they want to increase, they then roll 3d6. If the outcome is higher than the current attribute score the player gets to keep that score.
2. Spell mastery (Magic User/Cleric only) - One time only
The player can pick one spell to "master", they are allowed to cast it one extra time per day. As well they can add an extra 1d4 damage to offensive spells, and 1d4 to any healing spells.
3. Lingering injuries & medical conditions
You have been in battles, but where are your war wounds? Roll 1d8
1. Scar across chin
2. Missing an appendage
3. Blinded in one eye
5. Back pain
6. Weakened immune system
7. Liver damage
8. Strange growth
4. Add an extra 10% (Thieves)
Player can add 10% wherever they choose to a specific thief skill (or skills)
5. Undead Turning Mastery - one time only
Add a plus 1 on any turning roll to a specific monster that you have mastery over (skeleton, wight, etc
6. Gain fame/infamy
Every level you move up you gain one point of fame/infamy (depending on your characters actions). In some cases this will change the course of conversations with NPCs, as per GM discretion. It may have an effect on hireling morale, monster morale, bargaining and charisma checks. You may also get harder and more lucrative quests.
7. Fighter weapon mastery - one time only
You have mastery over one weapon, which allows you to have 2 attacks per round with that weapon
8. The gods are watching (once per level)
Once per level you are allowed to change any dice result to a critical roll.
9. Altered appearance
1. Your height has decreased
2. Your weight has increased
3. Your weight has decreased
4. Your hair is graying
5. Your hair is balding
6. Your beard has gotten longer! (alternatively you have started to grow a beard. This counts for dwarven females as well).