Depends...what do you like?
For some of the best pastries and deserts go to Mike's Pastries in the North End (Hanover St)
For a nice casual lunch, stroll through Quincy Market, (if you don't mind tons of people everywhere)
If you want to be seen and the weather is nice, try lunch on Newbury st (Stephanie's is propbably the best place)
For great Italian Food, also go to the North End, but for less money and people Papa Razzi near Copley is a cool Bargain.
As far as drinks go, there are bars everwhere.
For more unique restaurants, try the south end.
For seafood, there's a great place near Quincy Market, but the name is escaping me right now.
If you have any more specific questions, feel free to PM me. I've lived in Boston for a while.