These days achievement-based rewards are considered awesome. You want more than just the grind and grind and grind to get itemX.
Specifically I'm contemplating along the lines of:
Finishing a level:
a) Without taking any damage.
b) Using only a single weapon.
c) Not picking up a single drop.
d) Using only vials.
e) Not killing any monsters. (clearly wouldn't work with all levels)
And there being a level-based reward.
Complete a Wolver den only using a single weapon? Not a big deal.
Complete a Haunted Passage with a single weapon? More difficult.