Rome - little time, intense travelling [closed]

Next week I am going to visit Rome at scientific conference. It will last for 5 days (Monday - Friday) from 9:00 - 17:00. So I won't have much time to see everything. I was reading some guides and topics about Rome but my situation is very specific as you can see. I have very little time and want to use extremely effectively. I am planning to stay in the hotel near main train station.

1. What should I see with highest priority?
2. Any least expensive shops/ markets where I can buy something to drink/eat?
3. Bars and restaurants where I want to try pastas, pizzas and local food with not too high prize?

Thanks for help and advices!