If you're not interested in touristy stuff, I'd say the best thing you can do is just walk around and explore. Spend a day or a morning in the East Village walk over the West Village for dinner etc. Go over to Williamsburg in Brooklyn and check that out. I wouldn't worry about what restaurants to go to etc. Your friend should be able to tell you what's good. Just stay out of of obvious tourist traps and you'll be fine. If you're staying for 5 nights you might want a get a 7 day unlimited MetroCard. You can ride the buses (though not express buses) and subway all week for $32. In what neighborhood does your friend live?