There are 2 main architectural points of interest in Avignon - Pont d'Avignon (which has a famous song to go with it) and Palais des Papes.

 The first is the remains of an old bridge over the Rhone.  While the Rhone may look sedate on your visit, it was (before flood control measures were put in place) one of the rivers most liable to vicious flooding in Europe.  This flooding washed away 18 of the 22 spans of this bridge, leaving the 4 you can see today.

 The Palais des Papes is what most people coming to Avignon have on their must see list.  It is indeed worth a visit, although you may want to do some research before you drop by.  It is hard to get the true scale of this building - to do so just get your friend to stand by the door and go to the far side of the square in front of the entrance.  As your friend becomes smaller you'll get the idea!