Quick answer: Munich Airport has a Visitors Terrace with telescopes, a kids' playground with a slide-equipped control-tower replica, an airport brewery, a casino, and — if your layover is long enough — quick access into Munich itself.
Inside the terminal
Visitors Terrace (Terminal 2, Level 05, reached via the glass Skywalk) — views over the apron and runways, telescopes provided, open daily 8 AM–10 PM
Kids' playground — culturally-themed jungle gyms, swings, sandboxes, and a Kids' Tower control-tower replica with a slide; a restaurant sits right next to it
Visitor's Hill — a 172-step climb to a panoramic viewing platform, €1 entry
Airport brewery and a casino, both inside the terminal complex
If you have several hours
With a long enough layover, the S-Bahn puts central Munich under an hour away — see our guide to getting to the city center. Just make sure you've cleared any visa/Schengen requirements and leave enough buffer to get back through security.