Since this couple is considerably better than average at saving money (in other words not spending to much).  I would recommend paying down non deductible debt like their mortgage as their marginal tax rate is above 50%.  This would mean they would need a no risk return of almost 6% to offset a mortgage loan of 4%.  RRSP contributions are also a reasonable option because of the high marginal tax rate .  Being debt free always feels great at any age!!