As light as a feather
Bright as a button
Cold as ice
Dark as night
Elegant as a swan
Fast as a cheetah
Gentle as a lamb
Happy as a clam
Intense as fire
Joyful as a child
Kind as a mother
Luminous as the moon
Magical as a dream
Nice as pie
Outstanding as a star
Proud as a peacock
Quick as a wink
Radiant as the sun
Strong as an ox
Tall as a tree
Unbelievable as a myth
Vibrant as a rainbow
Warm as sunshine
X-cellent as a champion (a bit of a stretch, but works!)
Yummy as a cupcake
Zesty as a lemon
Let me know if you'd like more examples or want to try a different letter!