When you go on a flight, there will normally be a baggage allowance specified in kilograms, usually 20 kg per passenger. Some airlines/flights may have a higher limit of 30 kg.
During the flight on some aircraft, there will be a TV screen showing flight data, which may well show you the distance from your starting point in kilometres, distance to destination in kilometres, altitude in metres (e.g. 10900 m), temperature outside in degrees Celsius (e.g. -55°C — it gets very cold outside at high altitudes), and groundspeed in kilometres per hour (this is the equivalent speed of the aircraft if it were at ground level, as the distance from your starting place to destination is measured at ground level).
Also, if you are flying to any country other than the USA, when you get there you will find that everything is in metric.
Virtually all countries (outside of the UK) have speed limit signs on roads in km/h. Even the USA has some speed limits in km/h, although the majority are still in miles per hour.
Ireland changed all its speed limit signs to km/h in January 2005, as well as road distance signs to kilometres. The USA has a few road distance signs in km, but most in miles. All other countries usually have road signs in km. Most countries follow the international standard designs for road signs (not the USA though) so you will find that most road signs look familiar, although the numbers will probably mean something different to what they mean in the UK, e.g. a red circle with black number 50 inside it will mean 50 km/h not 50 mph.