You know I can't endorse that ;-)

There are an awful lot of places to visit there. The region around Banská Štiavnica and Zvolen might not be the best in spring -- in early spring, everything is just brown -- but maybe from late April.

Some other places you should not miss if you pass close:

• Oravský hrad, easily the most impressive castle you'll find there, in the romantic valley of the Orava river. (I just learnt from Wikipedia that the 1922 Nosferatu was filmed there!)

• The valley of the Váh river upriver of Žilina (narrow and deep, with more castle ruins).

• In the East, if you get to the Slovenský raj region ( = Slovakian Paradise;between Poprad, Spišská Nová Ves and Dobšiná), leave behind your car, buy a tourist map, and walk across it for at least a day. Beyond the obvious choice of the chasm of the Hornád, where you shouldn't miss climbing up to the rock outcrop named Tomašovský výhľad, follow at least one of the smaller creeks.

• Further Northeast, there's Levoča with its historical city core.