generally, ISO200 should be suitable for most situations. ISO100 would be good, provided if u are going to a place with good sunshine and weather. if not, u might need to use flash. however, for my personal preference, i usually carry ISO100 film when i use my wife's pentax on our trips overseas. we both like the sharpness of the prints when we developed them.
if you tend to spend a lot of times indoors (e.g museums, castles, churches etc), a ISO400 film might be a good idea, if u dun intend to pack an external flash along. helps you capture shots in low light without a tripod most of the time. if not, u might have to think abt lugging a lightweight tripod along for the trip.