My first reaction was South America...others beat me to it. Aus/NZ would be my next look. Also possibly South Africa, although I know some airline vouchers require their own metal the entire way (as opposed to merely ticketed on that airline).

I avoid Europe in the summer, but I do want to someday spend a month in Scandinavia and I suppose I'll want to do it in the months with more daylight. So maybe there, although it'd be a pricey trip for 2-4 weeks with kids, and perhaps not one the kids especially love.