your music is (probably) in ~/Music/iTunes/ and your photos are in ~/Pictures/iPhoto/.
Basically, if you just copy your home folder over to your external hard drive, you should have 90% of anything you consider important. (that would cover documents, music, photos, email, bookmarks, preferences, etc.)
Throw in occasional apps, or documents etc. that you may have saved outside your home folder, and you should be fine.