Your best bets are a Roth IRA or traditional IRA. If you roll it to a Roth, you will have to pay taxes on the amount you roll over (unless it was a Roth 401k), however what is in the Roth will grow tax free and it will be tax free when you withdraw. With a traditional IRA, you won't owe taxes on the money now but will pay taxes when you withdraw.
You won't be able to withdraw this money until 59 1/2 years of age without paying a penalty, the same goes for your current 401k. If you take the money (for mortgage, other investment, etc.) and don't roll it over to a qualified account, you will owe taxes on it plus a 10% penalty. So you will only get between 60% and 70% of its value.