Solo, Spandau Citadel (Zitadelle Spandau) works.
The moated star-fort and the Juliusturm climb are a genuinely atmospheric, kid-pleasing change of pace, with a quirky museum of 'unloved' Berlin monuments inside. The catch: it's far out in Spandau, niche, and not worth the trek unless you've already done the centre or have history-loving kids in tow.