In my opinion, this is the best option. I have done many things for my indoor cats over the years, but by far, the best thing I have ever done for them was to give them 24/7 access outside in a safe manner. Cats and dogs learn quickly how to use the pet door and it puts them in control of when they go in and out. Plus, as we all know, cats like to be where they're not. This way, it leaves you totally of out the equation.
It also opens up the opportunity for you to put either some or all of your litter boxes outside.
Do you already have a pet door?