What can I do in Lima Peru for a day?

I'll be in Peru working at an orphanage in Huancayo for a month. But I'll be stuck in Lima for a whole day, what is a "Can't Miss" attraction that I should go see while I'm there? I'll be arriving at the airport in the morning, then getting on a buss to Huancayo in the evening. Don't wanna be bored all day. :)