Completely under-appreciated for his contributions to mankind in most places, the inventor Nikola Tesla is actually everywhere in Belgrade (currency, street signs, statues, etc.). So is a museum with interactive exhibits, working models of his creations (like the AC motor) and personal effects--such as letters from scientists like Albert Einstein.
Though it's not a huge museum, there's even a working Tesla Coil which a guide demonstrates during a tour; visitors can hold a fluorescent light which becomes illuminated wirelessly by man-made lightning (pictured above).
If you have kids they'll learn a lot (so will you), and if not, this museum will make you feel like a kid again, full of imagination and possibility. Entry ticket costs 300 Serbian Dinar (about $4 USD) and includes guided tour (with time to wander afterward too). Closed Mondays. Check out tesla-museum.org.
Best place ever to get your geek on.