Zagreb (25 minutes east) — the Croatian capital is the obvious companion to Samobor. Combine a morning in Samobor with an afternoon in Zagreb, or come to Samobor as a day trip from a Zagreb base. The Upper Town, Dolac market, and the city's excellent café culture are all worth your time.
Žumberak-Samoborsko Gorje Nature Park (begins at the town edge) — for a longer half-day or full-day in the hills, push deeper into the park beyond the easy lower trails. Remote villages, hidden chapels, and quiet forest paths reward those with a car and a decent map.
Karlovac (about 50 km south) — a pleasant riverside town built on the confluence of four rivers, with a star-shaped 16th-century fortress layout still visible in the street plan. The old town is compact and walkable, and there is a good open-air "water castle" attraction at the river junction.
Plitvice Lakes National Park (about 2 hours south) — Croatia's most visited attraction and a UNESCO World Heritage Site, with cascading turquoise lakes and waterfalls in a forested canyon. It is too far for a casual half-day, but makes an excellent full-day excursion from Zagreb, often combined with an early-morning stop in Samobor on the way back.