I'll give my usual piece of advice - even if I gave the names of my favs, they should find their own. Avoid the touristy streets. Talk to the owner of the B&B or the concierge of the hotel. There are hundreds of great places everywhere. They would find something that struck their fancy long before they got to the restaurants I recommend. Stop in and look at menus. Mussles are just going out of season, but always excellent in Amsterdam. Gotta try pancakes. They have some brunch shops along Prinsengracht (I believe this is where the Anne Frank museum is) that are quite nice. If they like cheese, the aged cheeses will make them weep with tears of joy.
Make sure they hit the Anne Frank museum. Be prepared for the inhumanly steep stairs in all the narrow buildings. The typical American attitude of everyone speaking English is actually correct for Amsterdam. They'll have no problems with language. Remind them that pot smoking is legal and the pubs/cafes are not suitable for younguns.
