I stayed at the Regent Warsaw Hotel for five nights for a work related campus. The rooms are elegant and clean, and the staff takes care of your every need. Also, the breakfast is great even by Norwegian standards.
The hotel is located a bit outside city centre, but getting around i Warsaw is easy and cheap. Take an uber. You'll pay half of what a taxi costs. Regent Warsaw Hotel is located next to the beautiful Lazienki park. I highly recommend taking a stroll through the park if you have som spare time.
Oh, and btw; the gym at this hotel is the best I've ever experienced! Truly an awesome hotel gym. The fact that the gym is next to the pool means that you'll be able to cool down after your workout is done.