I would say play all those types of music around them everyday.
Ban the use of the iPod or MP3 player with all their favorites on for an hour or two in the home.
I have picked up lots of likes in music just by having it as background. Then after hearing it as noise I started to listen. Now I have a much wider range of musical likes than I did as a teenager.
Don't force one of your favorites on them however, that may backfire. Use gentle persuassion. Ask questions, point out similarities between their favorites, maybe they enjoy a modern cover of an older classic, encourage them to compare the two. What changed between the original and cover, what was kept the same.
The key would make the discovery fun. Young people are meant to have different tastes to the older generations, that is progress. But sometimes the old folks got it right too. :)