Hotels in walking distance to the University of Liege, Rectorat (first day) in the city centre (~ Opera): there are many options going from apartments, small boutique hotels and the larger ones: Pentahotel, Van der Valk Congres (other side of the Meuse, nice walk of about 10' in the morning), Selys.
Hotels convenient for transport, near Guillemins are a good option (late arrival, easy bus connection to center and campus): Ibis Styles or the Yust hotels as probably good/safe options as they are brand new and part of chains. This is the easiest in terms of transport, but requires a 20' walk to go to the Rectorat; for the second day, there are the buses in front of the hotel.
Traveling to Sart-Tilman Campus (second day):
For the second day, the best option is to take the bus from Guillemins station (48 or 58) and stop at Polytech. Taxis are available for around €25 - €30. For bus service https://www.letec.be/ctronic tickets are available with the app installed. If traveling by carto the Campus there are parking places available right next to the building (in Allée de la Découverte).
Arriving to Liege:
for almost every one, and especially UK, France and Netherlands, the best is to go to Bruxelles-Midi, either from the airport (Zaventem) or direct by train. There are fast connections from UK (Eurostar), from France/Netherlands (Thalys) and Germany (ICE), next to regular trains. From Bruxelles-Midi, there are direct trains to Liège Guillemins once an hour (Liège-Welkenraedt, about 1h). For tickets : https://www.belgiantrain.be (NL/FR/EN). Belgian train tickets are not on reservation - they are valid for one ride during the whole day. Train conductors accept electronic tickets (obtained via mail, or a dedicated app), no need to print if a bar/qr code is present.
another option is Brussels-South (Charleroi) airport. From there, you there is a bus shuttle 1 (6 euro, buy ticket before boarding on electronic distributor, every 30') to Charleroi-Central train station; from there you have a direct train to Liège Guillemins (Liège-Herstal, 1h30 travel) every hour.
for German participants, there is in fact a much better option since the ICE to Brussels (from Cologne) stop at Liège Guillemins.
for people staying in the city center, there are a few small stations in the city itself. In particular Liège Saint-Lambert is close to the Opera. You will have to change trains in Guillemins.