The best place to check on the opening hours of tourist sites is usually their official websites. Policies can change from year to year, so people's reports of what they experienced in the past may not be accurate reflections of what will take place next year.
https://www.auschwitz.org/en/visiting/opening-hours/ -- Auschwitz closed Easter Sunday
https://www.wieliczka-saltmine.com/individual-tourist/useful-information/ticket-prices-and-visiting-hours -- Salt mine situation unclear; see below.
Unfortunately, I don't see an opening-days policy on the Wieliczka Salt Mine website, and ticket availability doesn't extend as far out as next Easter. A different, commercial-tour website says the mine is closed on Easter Sunday, but I wouldn't consider that as absolutely definitive.
The eastern Czech university city of Olomouc is very attractive, though it might be quiet with the students (I assume) out of town for the holidays. Rick's video about Czechia outside Prague may be helpful to you, though it was released in 2008. You can watch it via this link: https://www.ricksteves.com/watch-read-listen/video/tv-show/czech-republic-beyond-prague
Quite a few small Czech cities have been recommended on this forum in the past, many of them possibly not too far off the southern driving route from Krakow to Prague, but I haven't visited any of them myself. Among them are Třebíč and Telč.
I assume this is a segment of a longer trip. Picking up a rental car in Poland and dropping it off in Czechia would probably involve a very high international drop charge.
