General Synonyms:
* Throughout
* In
* While
* Whilst
* Within
* Throughout the course of
More Specific Synonyms:
* For: (Emphasis on duration) - "He worked for three hours."
* Over: (Emphasis on a period of time) - "The project was completed over a year."
* Through: (Emphasis on a complete process) - "We'll be working through the weekend."
* Amidst: (Emphasis on being surrounded by something) - "The concert was held amidst a blizzard."
* In the midst of: (Emphasis on the middle of something) - "The news came in the midst of the celebration."
Other Options:
* At the time of: (Emphasis on a specific point in time) - "At the time of the accident, there was heavy rain."
* Throughout the: (Emphasis on an entire period) - "Throughout the summer, we enjoyed picnics in the park."
The best synonym for "during" will depend on the specific context of your sentence and the nuance you want to convey.