Nice. I'm a Styrian, but not from the Maribor area… If' you're willing to go to Celje, check out it's museum.
But since you're in Maribor, there's Pokrajinski muzej Maribor
The town of Ptuj isn't far from Maribor and has a lovely museum as well. It's worth a look.
Sadly, there aren't many places with stuff from old Slavic times… The're lots of medieval places, if you're interested in. Like Stari grad Celje, which is the biggest medieval castle in Slovenia
A pitty you're coming in winter, there are more events during the year.
I can think only of Rifnik hill near Šentjur. You can see there some ruins also from these times but I don't know if the're any guiding tours. Maybe you can find any info here:
There are numerous castles in the area and towns like Maribor, Celje, Ptuj and others have lovely old town centers. If you have the chance to go to Ljubljana, check out Etnografski muzej.
I hope I could help, it'd be easier if you could tell me what preciselly you're interested in. Like I said, foundings from the first Slavic times are rare. But there are plenty of medieval cites and foundings from the Roman era.