1

They're all worthwhile destinations. Still smaller:
near Maribor: Ptuj
near Ljubljana: Skofja Loka.
I heard bout a fabulous trainride through the Julian Alps, a.o. Bled via Bohinj Lake to Most na Soci

2

I suppose you already know all charming small towns of Burgenland, Styria, Carinthia, Lower Austria, Upper Austria as well as those in the Hungarian neighboourhood

3

Graz is worth seeing - it's a really beautiful small town, but there's probably only a day or two's worth of things to see - enjoy!

4

Surprised you put Bled - it's a bit off your route and really only a tiny village - apart from the lake I don't think there's much there. Ptuj as somone above mentioned is a good trip from Maribor (5o minutes by train, and you can get on to Zagreb without backtracking to Maribor), Graz and Klagenfurt are pretty, but the latter is not worth more than a day in March (later the lake is warm enough for swimming). From Klagenfurt to Maribor you'll have to go via Ljubljana. You might want to look at Eisenstadt, a small county town in Burgenland, Haydn's place of work, with a palace and some good vineyards around (though March is a bit early for the outdoor heurige). You can get from Eisenstadt to Wiener Neustadt by bus to pick up the main southern railway to Graz and Klagenfurt. Travel between Graz and Klagenfurt is by best by railway-operated coach, every two hours and takes two hours. The train link invloves changing at Bruck and takes up to 4 hours. You could instead cross by train or bus (changing once either way) from Eisenstadt to Sopron in Hungary, a pretty little town, with a rail link to Wiener Neustadt.

5

If you want to go southeast of Vienna I recommend Pecs in Hungary. It is a charming small town. Also going in direction of Graz and Slovenia, a tour on the scenic "Steirische Weinstraße" (Styrian route of the wine)may be of interest. It is near the Slovenian border and includes quiet villages like Gamlitz or samll towns like Leibnitz. You can get there from the train border station Spielfeld.