Formal:
* In those days
* At that time
* Formerly
* Previously
* In the past
* Historically
* Bygone days
Informal:
* In the old days
* Back in the day
* Once upon a time
* A long time ago
* Years ago
* Way back when
More specific:
* In the 1980s (if referring to a specific era)
* During my childhood (if referring to a personal time period)
* Before the internet (if referring to a technological shift)
Ultimately, the best alternative depends on the specific context of your sentence and the tone you want to convey.