General synonyms:
* Conceal
* Mask
* Veil
* Camouflage
* Cover
* Hide
* Dissimulate (to hide one's true feelings or intentions)
* Pretense (a false appearance)
More specific synonyms:
* Costume (for hiding one's identity or appearance)
* Alias (a false name)
* Incognito (to hide one's identity, especially from public recognition)
* Make-up (for altering one's appearance)
* Deception (for deliberately misleading someone)
The best synonym for "disguise" will depend on the specific situation and the meaning you want to convey.