(I split this part from Elektriktoad's post on non-mana Level up costs, since it seems to be worthy to discuss as a separate route from tapping to level up.)
Level-up triggers are an idea taken from some of the Mondombre submissions and Antny's Spelltrained/Battletrained mechanic:
Varaka Bully 1R
Creature - Warrior (1/1)
Level Up - Attack (whenever this attacks, put a level counter on it at end of combat)
LEVEL 1-2
First Strike, 2/1
LEVEL 3+
Double Strike, 3/1
Charms Pupil 2G
Creature - Wizard (2/2)
Level Up - Cast an enchantment spell (whenever you cast an enchantment spell, put a level counter on this)
LEVEL 1-2
Hexproof, 3/3
LEVEL 3+
Hexproof, 4/4