General antonyms:
* Bold
* Daring
* Reckless
* Impetuous
* Rash
* Careless
* Irresponsible
* Imprudent
* Foolhardy
* Adventurous
More specific antonyms depending on the context:
* Hesitant: (opposite of cautious in terms of taking action)
* Paranoid: (opposite of cautious in terms of being overly fearful)
* Trusting: (opposite of cautious in terms of being skeptical)
* Confident: (opposite of cautious in terms of being uncertain)
The best antonym will depend on the specific situation and the nuance you want to convey.