When we were stationed on the UK, we spent a week in Oberammergau twice with side trips to Salzberg and Neuschwanstein in the summers. We also went to Venice for a week during the off season - February, I think.
I think the best thing about homeschooling would be being able to travel in the off season.
Yes I've traveled a lot with my children, but mostly around my own country so I have no suggestions on where to travel abroad unless you come here.
We're planning a trip to Japan hopefully one day with the children as our abroad trip. The children are learning Japanese and we're working hard to save up and plan. I do have a friend in Canada that's trying to get us to visit her, so we may make it up there sometime too.