bright, sunny, prosperous, merry
熙 centers on radiant warmth and the flourishing that comes with it: bright sunlight, prosperity, and joyful merriment. The idea connects light, growth, and happiness as natural outcomes of a sunny, thriving state.
熙 combines 灬 (fire) with 巸, which likely contributes the sound and the idea of brightness or joy. The exact historical development of 巸 is uncertain, but the character as a whole expresses light and warmth.
The bottom 灬 is fire, and the top 巸 looks like a face (口) under a shelter (匚) with a curved line (已). Imagine a cheerful face basking in the warmth of a fire, making everything bright, prosperous, and merry.
For キ, picture a key (キー) that unlocks a bright, sunny day. The key turns, and suddenly everything is prosperous and joyful: key -> キ.