We will be staying in this Poznan Hostel for the first week of our trip.
This hostel does have internet access and linens. We do not have additional information on our Gdansk hostel yet, so it might be wise to bring one of your own towels.
Beds are provided with a fitted sheet and a duvet. Since it will be warm, you might consider just bringing a light sheet...in all that extra room in your suitcase. ;-)