Aside from some dungeon style decorations and non-D&D gifts here is what I came up with for my wife's surprise b-day party.
Sparkly pink dice that fit her character
Potions that will grant temporary buffs if the party decides to drink them
*L to R - Potion of Haste, Storm Giant Strength, Barkskin, Invulnerability, Flying, and Fire Breath.
And a cake with her character (or near enough!)