Interesting and difficult 'scientific' problem for geographers. We have a lot of beautiful landscapes in the north-eastern part, but in total I think they are a bit similar to the landscapes in Lithuania.
From the hills, I would recommend the vicinity of the Vistula Lagoon (towns of Frombork, Tolkmicko, Kadyny) with a view of the Vistula Spit - a bit like the Curonian Spit, but the land part is probably steeper than the vicinity of Palanga. Hilly (+ lakes) are also Kashubia and the closest to you - Suwalkai Region. It's just that I see similarities to Lithuanian landscapes...
Unfortunately, the mountains are far away, and the closest ones (Świętokrzyskie Mountains, around Kielce) are honestly not particularly mountainous.