I'd certainly recommend the Hilton in Frankfurt very highly. It is located near the center of the city (a long block from the Hauptwache). See the following thread for more details:

http://www.flyertalk.com/forum/Forum57/HTML/000268.html

In addition, if you can change to be there during the April 19 - 26 period, you may still be able to get a "point stretcher" award.

As to what to see, etc., be sure to eat at the Haus Werthym near the Römer. Depending on age and interests, you might want to visit a "lokal" in Sachsenhausen in the evening - apple wine and students.

Finally, if you can go during the run-up to Lent, be sure to check for Fasching (like Mardi Gras, only more so) parties in Frankfurt and near-by Mainz.

[This message has been edited by Counsellor (edited 01-15-2000).]