General:
* Busy
* Fast-paced
* Frantic
* Chaotic
* Harried
* Rushed
* Frenzied
* Feverish
* Whirlwind
* Tumultuous
More formal:
* Intense
* Stressful
* Demanding
* Overwhelming
* Exhilarating (if used in a positive sense)
Informal:
* Crazy
* Wild
* Nuts
* Non-stop
* Go-go
The best synonym will depend on the specific context in which you are using the word.
For example, "hectic" can describe a busy schedule, a fast-paced lifestyle, or a chaotic situation. Consider the specific nuance you want to convey when choosing the best synonym.