General antonyms:
* Calmly
* Indifferently
* Apathetically
* Boredly
* Dully
* Monotonously
Antonyms specific to a situation:
* Anxiously (if excitement is replaced by worry)
* Fearfully (if excitement is replaced by fear)
* Sadly (if excitement is replaced by sadness)
* Slowly (if excitement is replaced by a lack of urgency)
* Hesitantly (if excitement is replaced by doubt or uncertainty)
Example:
"He was excitedly waiting for his birthday, but his sister was indifferently looking at the presents."
The best antonym will depend on the specific meaning you are trying to convey.