I lived in Warsaw for six months on a study abroad program and have visited a few times since then. It's a fantastic capital city as it's small enough to conquer, but big enough that there's always something new to discover. I highly recommend taking trams, they're slower than the metro, but above ground so you can see the city better. Take a nice stroll down the whole length of Nowy Swiat all the way to the old town. You won't regret it as the street is full of beautiful architecture!
Insider Tip:Make sure you see the Uprising Museum! There's also a fantastic pierogi restaurant in Plac Zbawiciela. It's off the touristy path, but still has an English menu.