Hi. I live in the south and all my hikes are in the mountains or their foothills. I've never been anywhere close to our borders with BY, LT and RU so cannot recommend anything. But I know that @matthew1 , @sandraa1 @browery and @racibo are northerners and would probably suggest many interesting places. The latter is a professional tour guide so I recommend getting in touch with him.
If however you can endure the long trip to the south of PL (borders with CZ, SK and partially UA), I will be happy to be your guide. Good thing about mountains - unlike the sea - is that there's no season end. I hike every Saturday and in fact have already planned long weekends in October (best month imho) and November (a bit of a lottery, but last year was fantastic)