Inhaling vapors from a potion of gain level produces a message like “You smell hard work” or “You smell elbow grease” (perhaps if hallucinating), granting you 10% of the experience points needed to advance to the next level. Monsters may be similarly affected.

Note that the 10% is calculated just based on the level thresholds and your own experience points don’t matter (only your current level). If you are level 5, with level 5 reached at 120 points and level 6 reached at 200 points, you will always get 10% of (200-120) = 8 points.

Add a level flag for the level being open to the sky or otherwise no ceiling. Effects of this include: falling rock traps do not generate, cursed potions of gain level do not work, scrolls of earth do not work; monsters that normally cling to the ceiling are unable to. Levels that would have this: lots of Quest levels, the Elemental Planes.

Blessed potions of gain level give you at least 75% of the experience points required to reach the next level, instead of being totally random.

Cursed potions of gain level work in Sokoban, but they always place you on the downstairs of the next level up instead of randomly, effectively letting you skip any level except the last one.

Dipping a spellbook in a potion of gain level turns it into a spellbook of some spell that is one level higher. You only get “Interesting…” if you try this on a level 7 book. Not determined whether this should count as polymorphing an object to break polypileless conduct.

You can only use cursed potions of gain level once or twice while carrying the Amulet. After a few uses, the Amulet will start to pull you back down as you rise up, wasting the potion.

Rename the potion of gain level to “potion of level up”.