oobi shares???
just kidding - the mortgage offsetting sounds good - you will save a lot in interest just in that time if you have the right sort of mortgage (we put all of our salaries into ours and redraw and we save lots of int just by having as much as poss in there each day).
term deposits can also give you a risk free and easy return but it wouldnt be that big on that amount of money and that timing - but a safe bet if your mortgage isnt suitable.