General negativity:
* Depressing
* Gloomy
* Bleak
* Dreary
* Grim
* Morose
* Cheerless
* Desolate
* Forlorn
* Wretched
* Miserable
* Uninspiring
Specifically about atmosphere or environment:
* Somber
* Stark
* Barren
* Austere
* Unwelcoming
* Oppressive
Specifically about mood or feeling:
* Dejected
* Disheartened
* Downcast
* Despondent
* Hopeless
* Dispirited
* Melancholy
Figurative or literary:
* Funereal
* Doleful
* Lugubrious
The best synonym for "dismal" will depend on the specific context of your writing.