I'll leave the attractions for others to answer. There are a lot of things to see there and you can't really go wrong even if you have no idea of what there is to see. You can on the other hand really screw your trip up by not having a room reserved.
Americans think they can pick up an empty room, but there aren't any empty rooms. Seriously! You could easily see lines of people at tourist information places looking for affordable rooms and those might be several hours away.
On my third trip to Amsterdam someone else was in charge of getting rooms for us and they mistakenly thought we could just show up and grab something. Unfortunately we didn't find this out until we were already there and it really messed up our weekend.
Since then I've heard more than one story of people mistakenly doing the same thing. Don't do it!!!