Thanks Gagravarr and exilencfc for the rail info. This is very helpful - I wondered why I wasn't finding too much and the availability was just dropping off at weird times. Makes sense that it's released seasonally, rather than on an X-months-out basis.

That said, I wonder if the best play here is to just blow most of a day (or an overnight) to take the train from BUD to KRK, then set forth with a rental from there southward into the mountains. That would give us at least 3 full days, with a rental, to explore the region of Slovakia we're interested in. According to google maps, even Kosice (the furthest we'd consider going) is only 250 km from Krakow. It would take us less time to drive there than we would lose in transfers trying to get there by transit.

Or, are there any local or LCC airlines that do a direct flight?

Looks like cars can be had from KRK for 3-4 days for a cost of about $200 USD, which coupled with 2 train tickets BUD-KRK would still be much less than the close to $1000 for a BUD-KRK rental. Thoughts on this? Particularly, does anyone know if there is an issue with taking a rented Polish car into Slovakia (and bringing it back)? I had found a copy of the AVIS T&C for each European country the other day...of course I can't find it now. I know that there's the matter of buying a Slovak road sticker or whatnot, but that seems trivial. And it seems like most of the restrictions were on one-way drops or taking cars into countries considered riskier than that of origin - and my understanding is that Slovakia seems lower risk than Poland in the eyes of the rental car company actuaries.