General synonyms:
* Sag
* Wilt
* Slump
* Hang
* Sink
* Fade
* Decline
* Languish
Synonyms related to specific contexts:
* For plants: Wither, shrivel, fade
* For spirits: Flag, decline, weaken, slump
* For eyes: Heavy, weary
* For clothing: Sag, slump, hang
* For physical posture: Slouch, stoop, bend
The best synonym to use will depend on the specific meaning and context of the word "droop" in your sentence.