Or use RegioKarnet which (unlike miniRegioKarnet) is also valid in KW trains, as well as some other regional carriers (but not all - be careful! for example, definitely not Koleje Mazowieckie), it costs about 90 zl, and it allows you to validate it three times, for three days, so essentially three 24h tickets (= ca. 30 zl / 24 hrs) - very similar to the InterRail passes, but the huge difference is, that it's valid in the POLREGIO regional trains as well, which cover most of the country. As an another perk, it can be returned even if you've used it partially, like for one or two days, then you get directly proportional fraction minus 10%. Therefore, it can be also used as a real 24hrs ticket, which is even cheaper then the above (but list of carriers honoring this ticket is a bit different in both cases - the one mentioned by
@rg177 includes Koleje Mazowieckie which this does not, but this includes POLREGIO which the other does not). I'm travelling on it (and the miniRegioKarnet, which is only for POLREGIO trains, so no KW and so on...) fairly often, and it suits me pretty good, even for some cross-country journeys (yeah, sometimes I am determined enough), as well as some very cheap international routes, for example to Berlin via Kostrzyn (RegioKarnet/miniRegioKarnet + Deutschland Ticket).