Schengen is a shared zone
Many EU countries share the Schengen rules—your 90/180-day tourist stay counts across the zone. The UK is not in Schengen; a London weekend uses separate UK entry rules.
Passport validity
Airlines and border officers often want 6+ months validity beyond your planned exit—renew early if you are cutting it close.
Onward travel
Some carriers ask for proof of onward travel—have a return or next-leg booking you can show if asked.
ETIAS (future)
The EU plans travel authorization for visa-free visitors—check current requirements before each trip; rules evolve.
Read official sources
Always confirm US State Department country pages and official EU/UK border sites before you book non-refundable fares.