Sighteeing in Frankfurt.?

Visiting Frankfurt,Germany 3 days June/July. Are there any "must see/do" places to visit. Also any day boat trips recommended. By miffycind...go and see tha main river and try to get on the biggest building in frankfurt, as i remember it is the "citybank" building, you'll love the view from up there!! It is great how the city is really modern, with all those huge glass buildings but on the other way, don't miss the old town, i mean the old buildings and streets, it's great how new and old are combined!! Don't forget the airport, the biggset in europe