Focusing on the lack of imagination:
* Realistic
* Practical
* Down-to-earth
* Hard-headed
* Clear-eyed
* Level-headed
Focusing on alertness and attentiveness:
* Alert
* Sharp
* Wide-awake
* Focused
* Attentive
* Vigilant
Focusing on a lack of romanticism or idealism:
* Cynical
* Skeptical
* Pragmatic
* Realistic
* Unromantic
Focusing on a lack of drowsiness:
* Awake
* Alert
* Vigilant
The best antonym will depend on the specific situation and the nuance you want to convey.