Woah. I've been to Liège a few times, but only passed through. All I've seen is Sclessin (stadium of Standard, the local football team) and the outgoing highway, which I followed biking on the emergency lane.
Other than that they have a nice new railroad station, an aquarium and supposedly a cultural revival for the last few years.
(
http://www.tripadvisor.nl/Attractions-g188655-Activities-Liege_Liege_Province_Wallonia.html)
Honestly I wouldn't consider Liège city-trip worthy (probably because I'm flemish and biased), but I'm sure you can spend a lovely day there.
