General:
* Sadly
* Sorrowfully
* Mournfully
* Wistfully
* Longingly
* Despondently
* Dejectedly
More specific:
* Pleadingly: If the tone is begging or imploring.
* Melancholy: If the tone is introspective and reflective.
* Waveringly: If the tone is hesitant and uncertain.
* Elegiacally: If the tone is mournful and lyrical.
The best synonym for "plaintively" will depend on the specific context of your writing.